A congregant raised the question as to the permissibility of scheduling the Bar Misva celebration for his son the night before the fast of Asara Be’Tebet, which is the day the boy will become a Bar Misva. As we know, the fast of Asara Be’Tebet begins at daybreak, early in the morning, and no restrictions apply the night before. (Even on the fast itself, the only prohibitions are eating and drinking; it is entirely permissible to take haircuts, shower, listen to music, and so on.) Seemingly, then, it should be permissible to host a party that night. On the other hand, one might argue that since the calendar date that night is already 10 Tebet, perhaps it would be inappropriate to have a celebration that night.
Interestingly enough, Tosafot, in Masechet Erubin (40b), tell of a wedding that was held on the day of Asara Be’Tebet, and the cup of wine was given to a child to drink, as the adults were obviously not permitted to drink. It seems that at one time weddings were made on Asara Be’Tebet, surprising as that might sound. Accordingly, Hacham Ovadia Yosef rules that when necessary, one may make a wedding on Asara Be’Tebet, and have the meal served after the fast ends that night.
Certainly, then, if there is room to allow making a wedding on the day of Asara Be’Tebet, we may allow a family to make a Bar Misva celebration for their son the night before, when the fast has not even begun, especially if that is the day the boy reaches Bar Misva age. We should also note that some Poskim allow making celebrations on the night of Shiba Asar Be’Tammuz, which begins the period of the Three Weeks. Other Halachic authorities rule stringently and forbid celebrations the night before Shiba Asar Be’Tammuz, but regardless, on the night of Asara Be’Tebet, when no prohibitions apply, and which does not begin a period of mourning, one may certainly host a Bar Misva celebration.
Summary: It is permissible to schedule a Bar Misva or wedding celebration for the night before the fast of Asara Be’Tebet. There is a dispute among the Halachic authorities as to whether a celebration may be held on the night before Shiba Asar Be’Tammuz.
