It is permissible to make a festive meal for the sake of a mitzvah. This includes singing and dancing at the meal, to the degree that is acceptable throughout the year. For example, one may prepare a festive meal for a brit milah (circumcision), pidyon ha-ben (redemption of the first born son), or a siyum. And one who regularly dances and plays joyous music at such meals may do so during the Omer period, because these celebrations are made for the sake of a mitzvah.
The same goes for hiring out musicians: if the local custom is to always bring in musicians for these mitzvah parties, one is permitted to do so during the mourning period of the Omer. Even though some poskim rule strictly on this matter, the halachah follows those who are lenient; after all, we are dealing with a doubt regarding a custom of mourning. If, however, it is not clear that people hire out musicians for such events, it is preferable to avoid doing so during these days.
It is permissible to bring a new Torah scroll into a synagogue, accompanied by music, singing, and dancing, as is widely accepted, because such revelry is mitzvah oriented.
The same applies to a bar mitzvah celebration held on the very day the boy reaches thirteen. That is, one may celebrate the occasion as one would throughout the year. When it is impossible to schedule the party on the day the boy actually becomes obligated in mitzvot, it is permissible to make a festive meal, but without music. If, however, [the organizers] make sure that someone completes a Talmudic tractate or an order of Mishnayot at the beginning of the party, they may supply music, as people regularly do at bar mitzvah celebrations. 1. The questioners expected a lenient ruling seeing that it is a mitzvah to bring joy to these people. I answered that it is forbidden, because such a party is [nevertheless] optional, and the disabled have to keep the customs of mourning, as well. In addition, they do not make a party for them every week, not even every month, so why schedule a party for them specifically during a period of mourning? ]
- See M.A. 493:1 and M.B. 3, regarding engagement parties, which are considered se’udot mitzvah to some degree. Also see above, sec. 5 and note 7. The authorities that permit musical instruments at se’udot mitzvah are: Iggrot Moshe, O.C. 2:95, E.H.E. 1:97; Meshaneh Halachot 6:109; Yechaveh Da’at 6:34. Those who forbid it are: Minchat Yitzchak 1:111, based on Da’at Kedoshim; see also Piskei Teshuvot 493:5. The halachah follows the lenient opinion when it comes to these laws. The author of a book called Shalmei Mo’ed (p. 454) writes in the name of Rabbi Auerbach that one may celebrate a bar mitzvah on a different day, as long as there is no band or dancing. I was once asked if an institution for the disabled is permitted to make a party with music and dancing [during Sefirah ↩