Attending a meal and joyous event during the year of Aveilus, past Shloshim 
*From our Sefer “The laws of Mourning” Volume 2 Chapter 23. See there for the full details of this subject!
The laws of attending a meal during the year of Aveilus is similar to the laws of attending a meal during Shloshim, although contains certain leniencies. For this reason, this subject has been repeated here under the laws of the 12 months, in order to include the applicable leniencies relevant after Shloshim. All Q&A that receive the same laws as within Shloshim have not been repeated here, and please reference to chapter 22 Halacha 15 for the relevant answer. Regarding the laws of attending a meal and joyous event during Shloshim, and its greater level astringency, please refer to chapter 22.
Due to the vast complexity of this subject, of an Avel participating in a meal/Simcha, we will introduce here a short-summarized version of the Halacha, of which all its complex details will be elaborated throughout the Halachas to come.
It is forbidden for an Avel for the passing of a parent to participate in a meal throughout the 12 months of mourning, including a Seudas Mitzvah, unless one of the following apply:
*Sephardim allow participating in non-wedding Seudas Mitzvahs even during Shloshim.
Avel for passing of relatives other than parent: It is permitted for an Avel over the passing of relatives other than a parent to attend a joyous meal without restriction after the conclusion of Shloshim.
Avel for passing of parent: A mourner over the passing of a parent may not participate in a joyous meal throughout the entire 12 months of mourning. After the passing of 12 months, he may participate in joyous meals and events without restriction.
Leap year: In the event that the year of mourning was a leap year, the two months of Adar count as two months, and after the passing of 12 months [which is one month before the Yahrzeit] he may participate in festive meals without restriction.
The general rule is that an Avel of a parent may not participate in any meal of any sort during the 12 months of mourning. However, the level of prohibition and cases of exception depends on the type of meal taking place, such as if it is a Seudas Reshus [a common meal eaten with others] or a joyous Seudas Mitzvah [i.e. a wedding] or a non-joyous Seudas Mitzvah [i.e. a Bris], or a get together with friends, or on the location of the meal, as will be explained.
Seudas Reshus-Party/meal of friends: The prohibition against participating in meals applies even against participating with a group of friends who are making a meal.
Seudas Mitzvah-Wedding: It is forbidden for a mourner over a parent to participate in any Seudas Mitzvah which contains joy during the 12 months, such as a wedding feast.
Seudas Mitzvah that does not contain Simcha: Some Poskim rule it is permitted for an Avel to participate in any Seudas Mitzvah that does not contain joy, such as a Pidyon Haben and Bris Mila. Other Poskim rule it is forbidden for an Avel to participate in a meal of a Bris Mila [or any Seudas Mitzvah even if it does not contain joy]. Practically, the custom is for a mourner for a parent to avoid eating in any meal of any sort, throughout the entire 12 months, if the meal is taking place outside his home. If, however, the meal is taking place inside of the home of the Avel, then the custom is to be lenient to participate in any meal that does not contain Simcha, such as a Seudas Bris Mila, and certainly other meals. However, by a wedding meal one is to be stringent even if it takes place inside the home. [In a time of absolute necessity, one may be lenient to participate in a meal even if it is taking place outside of his home, with exception to a wedding meal. Furthermore, 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.]
Seuda is dependent on participation of the Avel: If the Mitzvah will be nullified if the Avel does not participate, such as if he is marrying off orphans Lesheim Shamayim, then he may participate starting from after the Shloshim. [Some Poskim rule that this allowance extends to any Avel whose participation in the wedding is desired by the Chasan/Kallah, and if he does not attend the Chasan/Kallah will be saddened. In such a case, the Avel may participate in the meal, and according to some may even remain in the hall during the music.]
Being invited out as guests, and inviting others as guests during Aveilus: Some Poskim rule it is forbidden for an Avel for a parent within 12 months to invite guests, or to be invited out as guests [for a meal]. However, on Shabbos, it is permitted to do so in an area that is accustomed to greeting Aveilim on Shabbos. Practically, on Shabbos one may be lenient in this matter [although not regarding participating in a public meal].
Inside the home of the Avel: If the meal is taking place inside of the home of the Avel then the custom is to be lenient to participate in any meal that does not contain Simcha, such as a Seudas Bris Mila, and certainly other meals. [This applies whether to a meal that is defined as a Seudas Mitzvah, or a Seudas Reshus.] However, by a wedding meal one is to be stringent even if it takes place inside the home.
Attending the meal on the basis of being Mishameish/waitering the meal: It is permitted in certain cases for one to participate in a meal during Shloshim, if he will waiter at the meal. See Halacha D for the full details of this matter.
Relatives attending Simchas: It is permitted for certain close relatives of the Baal Hasimcha to participate in Simchas during Shloshim, as explained in Halachas E-I. See there regarding the relevant Simcha and the exact ruling for which relatives may participate, and under what circumstances.
A mourner for a parent may not participate in any meal of any sort, throughout the entire 12 months, if the meal is taking place outside his home. If, however, the meal is taking place inside of the home of the Avel, then he may participate in any meal with exception to a wedding meal. [In a time of absolute necessity, one may be lenient to participate in a meal even if it is taking place outside of his home, with exception to a wedding meal. Furthermore, some Poskim rule that even initially it is customary to be lenient to participate in a Seudas Mitzvah after Shloshim. There are also certain exceptions afforded to relatives of the Baal Hasimcha, and one who will waiter at the meal, as will be explained.]
In those cases that the Avel is permitted to participate in the meal, may he do so even if there is music playing?
May children below Bar Mitzvah who are in Aveilus participate in a Simcha?
 391; See Pnei Baruch 20; Nitei Gavriel 2 16-34
 Michaber 391:2; Moed Katan 22b
 Michaber 391:2; Moed Katan 22b; Admur 128:55
 Michaber ibid; Rosh in name of Ravaya; Shach 391:2; 395:3; Taz 391:1 “All Aveilus ends after 12 months even in a leap year, and those that continue the mourning customs into the 13th month have no source and are certainly doing a worthless act”; Bach 391; See Nekudos Hakesef 391:1; See Beis Hillel 391
 See Aruch Hashulchan 391:7-10 for a general overview of the opinions in Rishonim regarding the meals under the prohibition. Some Rishonim rule that only a Seudas Reshus is forbidden while a Seudas Mitzvah is permitted. [Semag Aveilus and Rabbeinu Tam; Riy; Rebbe Yehuda Hachassid, mentioned there; 1st opinion in Rama 391:2; See Aruch Hashulchan 391:7] Others rule on the contrary, that only a Seudas Mitzvah is forbidden. [Raavad; 2nd opinion in Rama ibid; See Aruch Hashulchan 391:10]
 Michaber 391:2 regarding Simchas Mereios; Shach 391:1 “This is a meal that friends make together in which they eat and drink together”; See Pnei Baruch 20 footnote 27; See previous footnote
 Rama 391:2; Tur in name of Raavad; Admur 128:55
 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]
 1st opinion in Rama ibid; Terumos Hadeshen 251; See Raavad Avel 6:7
 2nd opinion in Rama ibid; Mordechai
 Rama ibid; Beis Lechem Yehuda 391; Derech Hachaim; Chochmas Adam 166:2; Kitzur SHU”A 212:1; Poskim in Nitei Gavriel 23 footnote 12
 Aruch Hashulchan 391:10
The reason: As in a time of necessity one may be lenient in Aveilus like the lenient opinion in Rama ibid. [ibid]
 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
 Rama 391:2; Tur in name of Raavad
 Aruch Hashulchan 391:5; Zera Emes 3:169; Poskim in Pnei Baruch 20:20 footnote 49
 Nitei Gavriel 20:10 footnote 29
 Taz 385:1; Bach 385 in name of Rabbeinu Yehuda; See Pnei Baruch 31 footnote 13 in name of Gesher Hachaim and Nitei Gavriel 33:4 footnote 8 and 33:7 footnote 13 for an analysis on this ruling and the fact it is omitted in other Poskim
 Vetzaruch Iyun from the fact that we permit participating in a meal that is taking place in one’s home, and hence why is one not allowed to invite guests to a meal in his home. However, seemingly there is a difference between a Seudas Mitzvah taking place in one’s home versus a Seudas Reshus, and a Seuda made by others in his home versus a Seuda that he is arranging. Vetzaruch Iyun.
 Beir Heiytiv 385:2 in name of Darkei Moshe
 Sheilas Yaavetz 2:180; Pnei Baruch 20:15; Nitei Gavriel 28:8
 Rama 391:2
 The reason: As one is not required to leave his home because of a Simcha taking place there.
 See Aruch Hashulchan 391:10
 The leniencies associated with relatives attending Simchas during Aveilus is based on two lenience’s found in the Rama; 1) One may waiter at a Simcha, although may not eat at the meal. [Rama 391:3] 2) If the Simcha will be damaged if one does not attend, he may participate even in the meal. [Rama 391:2 regarding Yesoma] Depending on the relation, at times the second leniency applies [parents and grandparents] and at times it is debatable if the second leniency applies or if only the first leniency applies [siblings] and at times only the first leniency applies [other relatives] and at times no leniency apply. [not a relative]. The first recording of an allowance for relatives is brought in the Maharashdam 202; See Nitei Gavriel 16 footnote 10 and in his introduction for a lengthy discussion on this topic
 Nitei Gavriel 23:27
 See Chochmas Adam 166:2 that by a Seudas Yesoma they may not play music; See however Chaim Sheol 1:84 and other Poskim in Pnei Baruch 20 footnote 12 and Nitei Gavriel ibid footnote 46 that one may participate even if music is playing. See also Poskim in Nitei Gavriel 16:10 footnote 13; Maharam Shick Y.D. 368; Shaareiy Teshuvah 551:3; Teshuvos Maimanis Shoftim 19; Shevet Halevi ibid
 Nitei Gavriel 16:10
 Pashut, as they do not have to keep Dinei Aveilus, as brought in Chapter 12 Halacha 1B!