Challa refers to the two loaves of bread (or matza) over which we say the ha'motzi blessing at Shabbat and Jewish festival meals.
The loaves must be:

• Whole, without significant parts missing.
• Made out of one or more of the Five Grains.

We are required to eat two meals on Jewish festivals, each preceded by kiddush, one in the evening and one in the morning. For both meals for each Jewish festival day (and all three meals on Shabbat), say ha'motzi over two complete loaves of bread, each of which is at least 1.3 fl. oz. in volume. 

Source of Saying Jewish Festival Kiddush
Some kiddushes are commanded by the Torah (d'oraita); the others are from Chazal (d'rabanan), as follows:
Kiddush d'Oraita

• First night of Jewish festivals
• Night of seventh day of Passover
• Night of Shmini Atzeret.

Kiddush d'Rabanan

• First day of Jewish festival
• Second night of Jewish festival
• Second day of Jewish festival
• Seventh day of Passover
• Eighth night of Passover
• Eighth day of Passover
• Shmini Atzeret day
• Night and day of Simchat Torah.

Source of Jewish Festival Kiddush Location
Saying Jewish festival kiddush at the place where you will eat your meal is a rabbinical (d'rabanan) enactment.
Jewish Festival Kiddush-Meal Quantities: Evening

• For evening kiddush for a first-night (d'oraita) Jewish festival, a minimum of 4 fl. oz. (119 ml) of wine must be blessed on and at least half must be drunk.
• For evening kiddush for a second-night (d'rabanan) Jewish festival, a minimum of 3.3 fl. oz. (99 ml) of wine must be blessed on and at least 2.0 fl. oz. must be drunk.
• For the evening meal, as on the first two Shabbat meals and for all required Jewish festival meals, a minimum of 1.9 fl. oz. (56 ml) of bread must be eaten within four minutes.

Eating Requirements for Jewish Festival Morning Kiddush
There are two separate eating requirements during the daytime. They may be combined (say/hear kiddush and start the main meal right away) but are often done separately (say/hear kiddush and then eat some light foods and beverages; the main meal is eaten later in the day).
Note Since eating and drinking requirements on all morning kiddushes (both Shabbat and Jewish festivals) are d'rabanan, the required beverage amount for morning kiddush is only 3.3 fl. oz. (99 ml) instead of the d'oraita 4 fl. oz. (119 ml) (which is required for kiddushes for Shabbat evening and all first-night Jewish festivals).

• Morning kiddush requires a halachically legal “meal” with these elements:
  • Blessing on a minimum of 3.3 fl. oz. (99 ml) of wine (or other beverage),
  • Someone's drinking at least half the beverage, followed by
  • Eating at least 1.3 fl. oz. (39 ml, or 1/6 cup) of some type of mezonot within four minutes. 
    Note If you drink at least 3.3 fl. oz. (99 ml) of wine within 30 seconds, you do not need to eat mezonot.
    Note You do not need to drink the wine or other kiddush beverage to fulfill “establishing a meal.” You may hear kiddush and then simply eat the required amount of bread or mezonot. This applies to Shabbat or Jewish festivals, evening or morning.
     

 

• The real meal (kovei'a se'uda) requires eating at least 1.9 fl. oz. (56 ml) of bread (or matza during Passover!) within four minutes. It must include enough food to satiate. 
  Note You can simultaneously fulfill the requirement to “establish a meal” and to “eat a meal” by eating one (the same) piece of bread.

Human life is valued in Judaism, unlike in some other religions. The Talmud says that if someone saves one human life (pikuach nefesh), it is as if he or she saved an entire universe. Almost all halachot may be overridden in order to save a life; the main exceptions are for Adultery, Murder, and Idol Worship—see Adultery, Murder, Idol Worship .

You must tear four vertical inches at the neck of your shirt (and jacket, too, if you wear one) when seeing the ruins of the Temple in Jerusalem for the first time in more than 30 days. You do not need to be close by; do kri'a even if you see the mosques that are on top of the Temple mount.

You may directly instruct or ask a non-Jew to do melacha d'rabanan for you on Shabbat, but only:

• To prevent a large financial loss.
  Note A large loss is subjective to the individual's actual wealth and also to that person's perception of what is a large loss. Consult a rabbi.
• For Shabbat needs.
  Example You may ask a non-Jew to unlock an electric hotel door or trigger an entrance door on Shabbat, since this is for a Shabbat need and the action is not prohibited d'oraita.
  Situation A new guest arrives and you need the space on the table occupied by your Shabbat candlesticks.
  What To Do You may not move the candlesticks, but you may ask a non-Jew to do so.
  Situation You left a non-muktza item that you now need for Shabbat on a tree branch (or a child left one in a tree house!).
  What To Do You may not climb the tree to retrieve the item, but you may ask a non-Jew to retrieve it. Example You may ask a non-Jew to turn on air conditioning.
  ExampleYou may ask a non-Jew (directly, no need to hint) to turn off a light if it interferes with someone's sleeping.

  • To allow doing a mitzva.
  • For other pressing (and certainly for life-threatening) needs, such as health, even if the person not sick.

  Note All of these are d'rabanan cases!

You may fold clothes on Jewish festivals by making a new crease, but only if there is already an existing one on the garment. If there is not a crease from before you used the garment, you may not make one.
Reason This avoids smoothing out clothing (a forbidden action on Jewish festivals and Shabbat).
 

Almost all mitzvot may be enhanced by:

• Making them beautiful (hidur mitzva), or
• Observing non-required stringencies (mehadrin).

Less than 1/48th missing is still considered a whole loaf. So if you only have two challot (or other loaves of bread) for Shabbat, you might be able to use one loaf twice, as follows:

• Wash your hands,
• Say ha'motzi,
• Cut off a piece that is less than 1/48th of the loaf, and
• Eat it.

REASON You may consider the remainder of that loaf as still being a full loaf and you may re-use it for your Shabbat morning meal.
NOTE If you have pieces of bread or other mezonot, you may:

• Cut off less than 1/48th of the loaf,
• Eat the additional pieces of bread to make a total of at least 1.9 fl. oz. (56ml), and then
• Re-use the same loaf for Shabbat morning.

Restrictions during the Nine Days before Tish'a b'Av are the same as for Three Weeks, plus:

• You may not eat meat or drink wine.
  Exceptions

  1. You may drink wine on Shabbat (but not on Rosh Chodesh Av or erev Shabbat.)
  2. You may drink wine for havdala (but ideally give the wine or grape juice to a child between ages 6 and 10).  
  3. You may eat meat or drink wine at a brit, siyum, or pidyon ha'ben.
• You may not wear freshly laundered clothes, or wear or buy new clothes.

• You may not wash yourself for pleasure.
  Note Showering or bathing to clean one's soiled body is permitted (except on Tish'a b'Av).  So you may bathe or shower during the Nine Days if you are dirty, sweaty, or smelly.
• You may not do any activities that involve luxury.
• You may not say she'hecheyanu except on Shabbat.
  Note Therefore you should not buy new fruits or new items that you will enjoy during the Nine Days.  But if you DO eat a new fruit or buy something new, you must say she'hecheyanu anyway.

After Yom Kippur, say the full havdala. 

Note You must light a candle from a flame that was burning since before Yom Kippur began. If you do not have one, skip the blessing on the flame.
If Shabbat coincides with Yom Kippur, you should light the candle from a flame that was burning from before Shabbat began. If you do not have one, you may light a new flame after Shabbat and Yom Kippur are over. Don't say the blessing on spices.

A frequently performed commandment generally takes precedence over a less frequently performed commandment, but ONLY:

• Regarding the order in which they are to be performed, and
• If there is no specific reason to do the less frequent one.

If you are only able to do one of several commandments, do the most important one. 

Who Is a Mourner

A mourner is defined in halacha as someone mourning during the 12-month mourning period for parents or the 30-day mourning period for the other five relatives (spouse, brother, sister, son, daughter). After 30 days, one is no longer a mourner for anyone but one's parents.

Mourners' Restrictions

If the mourner goes about business as usual, it may show he or she doesn't care about the close relative who died. The mourner should ideally not want to do these things. The mourner honors the dead person by refraining from pampering him/herself and refraining from going about his or her life as usual.

Public Meals

A mourner may not attend a public meal for any purpose. For example, if the mourner attends a lecture or Torah class at which food is being served, he or she may not eat the food. This only applies to sit-down meals; snacking is permitted.

Siyum/Brit/Bar Mitzva

After 30 days after a parent's burial, a mourner may:

• Attend a siyum or bar mitzva and eat there.
• Attend a brit but not eat there.

Note If there is music (live or recorded), the mourner must leave.

Weddings

A mourner may not eat at a wedding and may not even be in the wedding hall after the ceremony took. The mourner may also not hear the music at a wedding.

Exceptions

• If the mourner is the parent of someone getting married, the mourner can fully participate in the wedding.
• If the mourner is the bride or groom, he or she must normally wait to get married until after shloshim/30 days.

Note If it is after shiva, but still during shloshim, consult a rabbi.

Kiddush and Shabbat or Festival Meals

A mourner may not publicly (noticeably) mourn on Shabbat or festivals so he or she may attend Shabbat or festival meals and kiddushes if he or she would be expected to attend. If the mourner always or routinely invites some person or a lot of different people on Shabbat or festivals, it is still permitted. If the mourner does not routinely invite some person or a lot of different people to a Shabbat or festival meal, then he or she may not, for his or her own enjoyment, invite guests for meals. However, the mourner is permitted to do so for other purposes (for the benefit of the invited person or people), such as kiruv or hachnasat orchim. There is no limit to how many guests the mourner may host.

The mourner may attend or host a sheva brachot in his/her home.

A mourner should not be invited to meals, even for Shabbat or festivals; but if he/she was invited, he/she may go.

Holidays

A mourner does eat at a Purim or Jewish festival seuda, since there is no mourning on Purim nor on any festival (except Chanuka).

Jewish Festivals: Medicines: When To Take
Medicine generally may not be used on the d'oraita Jewish festival days.

Jewish Festivals: Medicine for Chronic Diseases
You may take medicine on Jewish festivals (whether d'oraita or d'rabanan) for:

• Chronic diseases, such as Parkinson's disease, or
• Any disease that affects your entire body.

Jewish Festivals: Medicine for Non-Chronic Diseases
You may take medicine on Jewish festivals for non-chronic illnesses, if skipping one day will prevent cure.  You may not take medicine for non-chronic illnesses if skipping a day will just delay your being cured (unless the disease affects your entire body--in which case, you may take the medicine).

Jewish Festivals: Medicines: When To Take: D'Oraita Festival Days
Here are the d'oraita Jewish festivals:

• First day of Sukkot, Passover, and Shavuot
• Seventh day of Passover
• Shmini Atzeret (8^th day of Sukkot)
• Yom Kippur
• Both days of Rosh Hashana (even though the second day is d'rabanan).

On these days, as well as on d'rabanan Jewish festivals, you:

• MUST take medicine whenever there is any question of a life-threatening disease or condition.
• MAY take medicine for a condition that affects the entire body (illness, weakness, etc.). Consult a rabbi if possible.
  ExceptionYou may not smear substances on skin UNLESS the illness is life-threatening, in which case even smearing is permitted.
• MAY use some medicines if only part of your body is affected by a non-life-threatening disease--consult a rabbi.

Jewish Festivals: Medicines: When To Take: D'Rabanan Festival Days
You make take medicines for any reason on d'rabanan Jewish festivals--even medicines not allowed on the first day of Jewish festivals--except:

• Medicines that you smear on skin.
• If the Jewish festival falls on Shabbat (which can only be second day of Shavuot).

To fulfill the two requirements for Jewish festival evening kiddush:

1. Make Kiddush
   Say, or hear, the three Jewish festival evening kiddush blessings/segments:
   1. Borei pri ha'gafen (if on wine or grape juice—preferred option), OR
      Ha'motzi (on two challot if you have no wine or grape juice, since no chamar medina is allowed for Jewish festival evening kiddush; see Jewish Festivals: Kiddush: Challot for Kiddush) AND
   2. Asher bachar banu mi kol am.... mekadeish Yisrael v'hazmanim, AND
   3. She'hecheyanu on all Jewish festival nights, except the last two nights of Passover.  So say she'hecheyanu on:
      • Both nights of Rosh Hashana (in or outside of Eretz Yisrael),
      • First two nights of Passover (1 night in Eretz Yisrael),
      • Both nights of Shavuot (1 night in Eretz Yisrael),
      • First two nights of Sukkot (1 night in Eretz Yisrael), and
      • Nights of Shmini Atzeret and Simchat Torah (same day in Eretz Yisrael).
      Note There is never any requirement on an indvidual to drink kiddush wine (except at the Passover seder), but the kiddush wine must be drunk by one or more persons.
      Note A woman who had said she'hecheyanu when she lit the Jewish festival candles does NOT say she'hecheyanu again if she makes kiddush for herself, even when making kiddush at the Passover seder.
      Note There is no need to eat a new fruit after saying she'hecheyanu on the second night of  Shavuot, Passover, or Sukkot. This is not comparable to Rosh Hashana, since the second festival night was instituted due to uncertainty of the actual date of the holiday, while Rosh Hashana is considered to be one single, long day.
2. Establish a halachic “meal” (kovei'a se'uda).
   For how to establish a halachic meal, see Introduction to Jewish Festival “Eating a Meal” Requirements.

Introduction to Missed Amida/Tashlumim
If you accidentally miss any amida, you may make up for it (tashlumin)--unless you intentionally missed it.

Amida: Errors: Missed Amida/Tashlumin: Regular Days
Missed Ma'ariv Amida
If you accidentally did not say the amida for ma'ariv:

• Say the normal shacharit amida the next morning with the other men in the minyan.
• Say ashrei.
• When the leader begins his repetition of the amida, say the amida along with him, word for word, including kedusha.
• After saying ha'el ha'kadosh, finish your amida at your own pace.
• If you are not with a minyan, say ashrei and then repeat the shacharit amida.

Missed Shacharit Amida
If you accidentally did not say the amida for shacharit:

• Say the normal mincha amida with the other men in the minyan.
• Say ashrei.

• When the leader begins his repetition of the amida, say the amida along with him, word for word, including kedusha.
• After saying ha'el ha'kadosh, finish your amida at your own pace.
• If you are not with a minyan, say ashrei and then say the mincha amida a second time.

Missed Mincha Amida
If you accidentally did not say the amida for mincha:

• Say the normal ma'ariv amida with the other men in the minyan.
• Say ashrei.
• Say the ma'ariv amida a second time. Skip modim.
• If you are not with a minyan, say ashrei a second time and repeat the ma'ariv amida.

Note At the next prayer service, say whatever is the correct amida for that later prayer service, even if it is not the same amida that you missed.
If you miss mincha on Friday, say the ma'ariv service for Shabbat and repeat that amida again.
Note Once the time for the next amida has passed, you may not make up the missed amida.
Example
If you missed mincha on Thursday, you may only say tashlumin for mincha as long as you may still say ma'ariv, which is daybreak of Friday morning.

Amida: Errors: Missed Amida/Tashlumin: Shabbat/Jewish Festivals
Even if you forgot to say a prayer service on Shabbat and Jewish festivals, say the next prayer service amida and repeat THAT amida to make up (tashlumin) for the one you missed--even if it is no longer Shabbat or the Jewish festival.
Exception
There is no tashlumin for musaf. However, you may say musaf until sunset, even if you already said mincha.
Note If the time for mincha has arrived (½ hour after halachic midday), you must say mincha before saying musaf (but if you could join a mincha minyan later, you may say musaf now).
Situation
You miss Shabbat mincha.
What to Do
Say ata chonantanu in ma'ariv, but only for the first time you say the amida, not the second time (which is tashlumin).
Situation
You forget to say ma'ariv on Saturday night.
What to Do
Say ata chonantanu on Sunday morning in the second amida (which is tashlumin).

If Doubt about Whether You Said Amida
If you are not certain whether you said an amida:

• On a weekday, assume that you did not say the amida and say it anyway.  Intend that:
  • If you forgot the previous amida, this makes up for it, and
  • If you did say the previous amida, the second one is a nedava (free-will “offering”).
• On Shabbat and Jewish festivals, do not say the amida twice as tashlumin.

            Reason You may not say a voluntary prayer service (nedava) on these days.
Men who intentionally delayed saying shacharit past the fourth halachic hour of the day still say that amida until midday but if they did not say it by midday, they may not say tashlumin. See Minyan: Keeping Pace: Shacharit Minyan.