Year of Aveilus:[1]

Some Poskim[2] rule an Avel of a parent during the year of Aveilus may not participate in a Seudas Bris Mila that is taking place outside of his home [even if he is invited to the meal[3] and even if the meal is dairy[4]]. Practically, the custom is for a mourner to avoid eating in any meal of any sort, throughout the period of Aveilus[5], if the meal is taking place outside his home.[6] [This however is with exception to certain cases, as explained below.] However, if the Avel is having a Seudas Bris Mila take place in his home, then he may participate in the meal. [In all cases, the Avel may participate in the actual ceremony of the Bris even outside of his home[7], including the light refreshments given afterwards[8], so long as he does not remain for the meal. One, however, may not participate while music is playing.[9]]

Bris of child:[10] If an Avel is having a Bris for his son take place during the year of Aveilus, he may wear Shabbos clothing until after the Mila. The Avel may participate in the meal even if it takes place outside of his home, if he is after Shloshim.

Mohel and Sandek:[11] If the Avel is a Mohel or Sandek during his year of Aveilus, he may wear Shabbos clothing until after the Mila. The Avel may participate in the meal even if it takes place outside of his home, if he is after Shloshim.

Mishameish: An Avel may work in the kitchen or in the waiter’s room during a Seudas Bris Milah. [He, however, is to eat alone in a side room.]

Some Poskim rule it is forbidden for an Avel to be a waiter by a meal that he may not participate in. Other Poskim rule it is permitted for an Avel to be a waiter by a meal, whether a wedding meal or Seudas Bris Milah and the like. Practically, the custom is to be lenient. [This applies even during Shloshim.  However one is to only resort to this leniency if he is a relative of the Baal Hasimcha. It is however absolutely forbidden to participate in the meal. He is rather to eat alone in a side room.]

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[1] Rama 391:2

Custom of Sephardim: The Sephardim are accustomed to attend any Seudas Mitzvah that does not involve Simcha, even during Shloshim. [Birkeiy Yosef 391:2; Poskim in Nitei Gavriel 31:1]

[2] Rama 391:2 [2^nd opinion in Rama and final ruling of custom]; Mordechai; Beis Lechem Yehuda 391; Derech Hachaim; Chochmas Adam 166:2; Kitzur SHU”A 212:1; Poskim in Nitei Gavriel 23 footnote 12

Other opinions: Some Poskim rule it is permitted for an Avel to participate in a Seudas Bris Milah. [1^st opinion in Rama ibid; Terumos Hadeshen 251]

Eating in a side room: See Nitei Gavriel 23:19 based on Rama 391:3 that a waiter may serve food at the meal and then eat next door.

[3] Poskim in Nitei Gavriel 23:7 footnote 13

[4] Maharam Shick Y.D. 366; Nitei Gavriel 23:11

[5] Meaning, within Shloshim for Aveilim of all relatives [Taz ibid; Shach ibid], and within 12 months for an Avel over a parent. [Rama ibid]

[6] Rama ibid; Beis Lechem Yehuda 391; Derech Hachaim; Chochmas Adam 166:2; Kitzur SHU”A 212:1; Poskim in Nitei Gavriel 23 footnote 12

Other opinions: Some Poskim rule that even initially it is customary to be lenient to participate in a Seudas Mitzvah after Shloshim if it does not contain Simcha. [Beis Lechem Yehuda 391; See Shach 246:27 in name of Maharam Mintz 119 regarding a Siyum Misechta, brought in Rav Akiva Eiger 391:1; Degul Merivava 391; Pischeiy Teshuvah 391:5; Beis Lechem Yehuda 391; Gilyon Maharsha 391; Chochmas Adam 166; Gesher Hachaim 21; Poskim in Nitei Gavriel 29:1 footnote 2; See Kitzur SHU”A 212 in Lechem Hapanim 1 who asks on Shach and Poskim ibid based on Rama ibid]

[7] Nitei Gavriel 23:10 in name of Ritva Kesubos 8; Chaim Sheol 21; Gesher Hachaim 21:8; Pnei Baruch 20:23

May the Avel sing Chazanus by the Bris? See Nitei Gavriel 23:29

[8] Teshuvah Meahava 3:413; Nitei Gavriel 23:28

[9] Pnei Baruch ibid

[10] Rama 391:2; Maharil

[11] Rama 391:3; Taz 391:3 that the Rama refers to one who is an Avel for a parent