The purpose of a se'udat mitzva is to honor the mitzva.

A se'udat mitzva is required for a brit mila, but the brit mila is still valid even if no meal is held. The minimum requirement for the meal is to eat at least 1.3 fl. oz. (39 ml, or 1/6 cup) of bread within four minutes.

If a brit mila is performed on Tish'a b'Av or other fast days, the meal (se'udat mitzva) is held after the fast ends. On a delayed fast day, the sandak, mohel, and father of the boy who is having the brit may eat after mincha.

A mourner may not generally enter a hall of joyous celebration and may not eat at any public meal. During the year of mourning for parents, you may not join any public festivities (even if it is not a simcha) that have a meal, including any meals celebrating a mitzva (se'udat mitzva) such as for a brit mila, wedding, or redemption of a son (pidyon ha'ben). After 30 days, you may attend a bar mitzva or a siyum meal, since a bar mitzva is similar to a siyum since the child's parent's commandment to educate his/her child in Jewish education has been completed.
ExceptionA mourner whose child is getting married, does attend the wedding and does eat at the meal with everyone else, even if mourning for a parent. He or she does not need to leave the room when music is being played. To attend the wedding of anyone other than one's child, regardless of who died, a mourner must eat alone and outside the main dining area.

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.

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).