Question: Is it halachically permissible to give presents to a chatan or bar mitzvah boy on Shabbat?
Answer: The Mishnah Berurah (Orach Chayim 306:33) prohibits giving presents on Shabbat and Yom Tov unless it is for the purpose of Shabbat or Yom Tov or l’tzorech mitzvah. Otherwise, it is considered like a business transaction which is prohibited on Shabbat and Yom Tov. In light of this prohibition, the Ba’er Heiteiv (O.C. 306:6) questions the validity of the common custom to give presents to a chatan on Shabbat.
But the Aruch Hashulchan (O.C. 306:17) contends that making a groom happy is a mitzvah, and therefore one may give him presents on Shabbat. This is especially the case in communities that hold a big Kiddush for the groom and bride the Shabbat after their wedding. Offering presents at such an occasion will certainly gladden the new couple.
In addition, the Aruch Hashulchan notes that giving presents is qualitatively different than transacting business. When one transacts business, one records the event in writing. People rarely record gifts at the time they receive them, however. The Aruch Hashulchan concludes by noting that “ein ba’zeh d’rara d’issura, u’minhag Yisrael Torah hi.”
The same argument could be made for giving presents to a bar mitzvah boy on the Shabbat of his bar mitzvah, as well. The party and event are means of instilling in the child the joy and importance of observing mitzvot. Receiving presents certainly makes the observance of mitzvot more enjoyable to a child.