212:1 A mourner is not permitted to eat at a festive meal celebrating a bris, a pidyon haben or a siyum (a circumcision, the redemption of a first-born, or the completion of a tractate, respectively), and especially not at a wedding feast, during the entire shloshim for most relatives and all twelve months for a parent. Twelve months are observed even in a leap year (which has 13 months).
If the festive meal is being held in the mourner’s home, he may eat, though he should act stringently when it comes to a wedding feast, even in his own home. If he is marrying off an orphan and his lack of participation may jeopardize the match, then the mourner may eat even if it’s not in his home. He may likewise wear Shabbos clothes after shloshim when mourning for a parent, and even during shloshim for other relatives.
212:2 A mourner is not permitted to invite others as guests, nor to accept an invitation. He may not send portions of food to others and they should not send to him during shloshim, or during the whole twelve months when mourning for a parent. Doing so on Shabbos depends on the local accepted practice; see 210:8