213:2 If a man's wife died, he may not remarry until three festivals have passed. This is so that the joy of the festivals will help him overcome his grief by the time he remarries. The purpose of this is that he shouldn't be thinking of the first wife when he is with the second. Rosh Hashana and Yom Kippur are not considered festivals for the purpose of this halacha, nor is Shemini Atzeres counted as a separate festival. If he has not yet fulfilled the obligation to have children, or if he has small children with no one to look after them, he need not wait until three festivals have passed, but he should nevertheless wait until after shloshim. A woman whose husband died must wait ninety days before remarrying (to ensure that she is not pregnant from her first husband).
213:3 If a person is preparing for his or her own wedding and one of the relatives of the bride or the groom died - even the father of the groom or the mother of the bride - nowadays, since there are other people who can help prepare what is needed for the wedding, we postpone the wedding until after the mourning period.