My brother is going to be 13 two days before my wedding. My whole family is planning on coming to New York for the wedding several days before, but this will present a conflict with the date/location of the Bar Mitzvah. My question is about the Halacha: When can we do his Bar-Mitzvah, before his Hebrew date or after? If it is before, it would be a whole week before. If it is after, it would be about three weeks after. What should we do?

Mazal Tov Mention his Bar Mitzvah on the day he turns thirteen with a small celebration and note this is the real Bar Mitzvah day, with all the Halachic ramifications; the Seudah on that day will be a Seudat Mitzvah. The party with all the family and friends can be after the date. If words of Torah will be said by the boy it will also be defined a Seudat Mitzvah. (Mishnah Brurah 225; 6)