Klausenberger Rebbe: Seudas Bar Mitzva, The Night Of The Bar Mitzva Or The Next Day
There are two minhagim regarding a Seudas Bar Mitzva. One is to make a Seudas Mitzva the night the boy turns 13 in order to celebrate his entering the Ol Mitzvos. Others do not make the Seuda that night but rather wait until the next day, after the boy has put on Tefilin.
The Klausenberg Rebbe in Divrei Yatziv (7:26) explains the reason for this. To become a Gadol Min HaTorah a boy must have Simanim of Shtei Saaros. While there is a Chazaka that a 13 year old boy has the requisite simanim, there is a question in the gemara whether this chazaka works for things that are D'Oiraisa (See Divrei Yatziv OC 45).
If we do not rely on the Chazaka for D'Oiraisa then the boy may not yet be obligated in Mitzvos Min HaTorah. Therefore it is better to wait until after he puts on tefilin and celebrate that occasion. Even if his tefilin is only a Mitzva of Chinuch M'Drabanan, still it is cause for celebration and the party is not in vain.
Important Note: We bring this tshuvah as a starting point for discussion and not to convey any halacha. We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.