The two loaves of bread on Shabbat reminds us of the double portion of mun we received in the desert. Even though one portion would have been eaten by Shabbat morning, we still use two loaves in the morning and two for se'uda shlishit as a reminder of the miracle.

You may take medicine on Shabbat for non-chronic illnesses, if skipping one day will prevent cure, but not if skipping a day will just delay your cure.
Exception  If the disease affects your entire body, you may take the medicine anyway.
 

When you have food mixed with non-desired substances, you may remove the non-desired ones by picking up the entire mixture and dropping away the non-desired elements.

Shabbat: Easy to Remove
If each unwanted element is easy to identify and remove, there is no issue of boreir. Consult a rabbi regarding what is halachically considered easy to remove.

Shabbat: Removing Cantaloupe Seeds
You may remove cantaloupe seeds only by shaking, not by scooping, them out.

Shabbat: Removing Lemon Seeds
You generally may not remove lemon seeds (pits) from food. However, if you are squeezing a lemon and some pits get partly squeezed out, you may:

• Shake the lemon in order to shake off the pits, or
• Use your hand to remove the pits from the surface of the lemon.

Once the pits fall onto food, you may use your hand to push the seeds to the side of the plate. But you may not use any instrument to do so, not even a non-specialized instrument such as a spoon.

Note If there are only one or two seeds and they are easily differentiated from the food, you may remove them by any means, except by using an instrument that is intended to separate food from non-food or from undesired food, such as a sieve, strainer, or slotted spoon.

You may pour excess wine from kiddush back into the bottle as long as there is more wine already in the bottle than what you are pouring back and as long as the bottle has been toveled.
If there is less wine in the bottle than in your glass, you must pour at least one drop of wine from the bottle into your wine glass or cup before you pour it back into the bottle.
 

To do Shabbat kiddush,

• Say, or hear, the Shabbat kiddush blessings/segments, and
• “Establish a meal” (kovei'a se'uda).

For details, see How To Do Shabbat Evening Kiddush  or How To Do Shabbat Daytime Kiddush.
NoteThere 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.

Light Shabbat candles wherever you will eat dinner.

Some communities have the custom of an onen's saying kaddish.
NOTE When a person's parent dies on or just before (erev) Shabbat or a Jewish festival, a daughter of any age should not be told until after Shabbat or the festival is over. A son should only be told if he is 6 years old and above and the custom in that community is to say kaddish as an onen.

You may not wash or hang up wet laundry on Shabbat (or Jewish festivals).  The halachot for drying laundry depend on whether you use a clothesline or a dryer:

Shabbat: Laundry: Clothesline
You may take down laundry on Shabbat only if it was dry before sunset on Friday, and only if you don't:

• Transfer the laundry from one halachic domain to another (hotza'a), or
• Give the impression that the laundry had been washed on Shabbat (mar'it ayin).

If laundry on a clothesline is still wet at sunset on Friday, the laundry is muktza and you may not take it down or use it during Shabbat. This is different from the case of a dryer.
Reason On the clothesline, there is no certainty that the laundry will dry during Shabbat (it might rain, it might be cold or cloudy...), so the person cannot have in mind that it will dry during Shabbat.

Shabbat: Laundry: Dryer
Laundry in a dryer (even if it was wet at sunset) that was turned on before sunset on Friday (or Jewish festivals) is not muktza, even if you do not intend to wear it.  You may remove the dry laundry from the dryer on Shabbat as long as no light goes on.

The Torah forbids cooking on Shabbat.   “Cooking” means making food edible by heating it to above 120° F (49° C).
Cooking includes:

• You may not make a soft food hard (such as cooking an egg).
• You may not make a hard food soft (such as cooking meat).
• You may not, in any manner, heat (to 120° F or above ) liquids that you will drink or foods with liquids—such as sauces and gravies--whether fully cooked or not. 

According to the Torah, you may eat food on Shabbat that had been placed on the heat source Friday afternoon but was not completely cooked by sunset.
ReasonNo action is being taken and the cooking will be completed by itself.
 
According to Torah law, food is only considered to be cooked if it has been directly heated from the heat source, such as a kettle on a fire or a pot on a flame (even if that utensil has been removed from its heat source). This is called a kli rishon.
Once you pour water from a kli rishon into a glass, the glass is a kli sheni. Some foods, such as an egg or tea, get cooked in a kli sheni (kalei bishul--easily cooked). These foods are forbidden by Torah law to be put into a hot kli sheni on Shabbat.

• You must have taken it off with the intention of replacing it, and
• You may not put the utensil down onto a surface; you must continue to hold the food (or the utensil) in your hand.
• The heat source must be covered.

There are two requirements for Shabbat daytime kiddush: Say or Hear Kiddush Segments/Blessings and Establish a Halachic Meal (kovei'a se'uda):
 
1. Say or Hear Kiddush Segments/Blessings
    You must say, or hear, the Shabbat daytime kiddush segments/blessings and someone
     must drink at least 2 fl. oz. (59 ml) of the kiddush beverage:

• Say or Hear Kiddush Segments/Blessings
  • Torah segment(s): V'shamru bnei Yisrael (even beginning from al kein).
  • Blessing over at least 3.3 fl. oz. (99ml) of drink:
    • Borei pri ha'gafen (if on wine or grape juice), OR
    • She'hakol nihiyeh bi'dvaro (if on other beverage/chamar medina).

• Drink at Least 2 fl. oz. (59 ml) of the Kiddush Drink

 
2. Establish Halachic Meal (Kovei'a Se'uda)
    You must establish a halachic meal (kovei'a se'uda) shortly after saying or hearing
    Shabbat morning kiddush by either:

• Drinking Wine--at least 4 fl. oz (119 ml) of wine (or grape juice) within 30 seconds, OR
• Eating Bread/Mezonot--at least 1.3 fl. oz. (39 ml, or 1/6 cup) of bread or mezonot within four minutes.

Note The second half of making kiddush, “establishing a meal” (kovei'a se'uda), can be fulfilled simultaneously when you fulfill the subsequent, separate Shabbat requirement for “eating a meal” but in that case, you must eat 1.9 fl. oz. of bread.
Note If you make, or hear, Shabbat morning kiddush on any beverage except wine or grape juice, you must also eat at least 1.3 fl. oz. (39 ml, or 1/6 cup) of bread or mezonot within four minutes to establish the kiddush meal. If you do not want to eat bread or mezonot, only drinking at least 4 fl. oz. (119 ml) of wine (or grape juice) within 30 seconds will fulfill all the kiddush requirements. 
Note If you have not fulfilled the kiddush requirements, you may not eat other foods, such as fruit or fish at a kiddush.
Note   Once you have heard kiddush and either eaten the required bread or mezonot or drunk the required 4 fl. oz. of wine or grape juice, you do not need to say or listen to kiddush again if you eat your actual meal later (except if you need to say kiddush for other people who have not yet heard or said kiddush).

As on Jewish festivals, the minimum total volume of Shabbat kiddush beverage that must be drunk--usually by the kiddush-maker (mevareich) but it may even be by several people combined--is a cheekful (m'lo lugmov), as follows:

• 2 fl. oz. (59 ml) within 30 seconds of beginning to drink for d'oraita cases such as Shabbat evening kiddush (as well as first-night Jewish festival kiddush and all havdalas), and
• 1.7 fl. oz. (50 ml) within 30 seconds of beginning to drink for d'rabanan cases such as Shabbat lunch kiddush (as well as first-day Jewish festival lunch kiddush and all second-day Jewish festival kiddushes).

Like Shabbat candles, Jewish festival candles should be lit only by one person per home. Priority order: wife; then husband; then children.
It is customary for each married woman to light candles on each festival even though she is not eating in her own home and even though her hostess is already lighting candles. She may light her candles at her hostess's house or at her own home (but if at her own home, she must see the candles are burning after dark if she lights there).
Girls should not be encouraged to light Jewish festival candles except when no parent can.
Single people should light Jewish festival candles in their homes if they will eat there.
 

A non-Jew is not permitted to observe Shabbat completely. He or she must do at least one violation. This is relevant for people who are in the process of converting to Judaism.
 

You may wear extra shirt buttons, sewn on to be used in the future if buttons fall off the shirt, outside an eruv on Shabbat, but not if you are already missing a button from your shirt and you plan to use one of those buttons in the future.