1

According to the Zohar Chadosh (Bereishis 15:4) the joy on the day one becomes Bar Mitzvah should be as great as one's joy on the day of his wedding. According to the reason provided there, it would seem that the same is true with regard to becoming Bas Mitzvah.

One should not celebrate a Bas Mitzvah in the style of those who are "modern," who celebrate it in the same manner of a Bar Mitzvah. Rather, to do something in a modest manner in school, similar to a birthday. And the parent's should buy her a nice present, etc.

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To do it (i.e., celebrate the Bas Mitzvah) only as part of a Mesibas Shabbos or a Melaveh Malkah.

Excerpts from letters of the Rebbe, printed in Sha'arei Halachah U'Minhag II, pp. 311-312

2

It is self-understood that I am not addressing here the general matter of celebrating a Bas Mitzvah, and it seems that this was also not your question. Merely that in light of the present condition of your community the question is solely about the time frame.

Although the following note is surely superfluous, still, as an added caution I wish to write the following, because of the indiscretions committed in many places regarding the ceremony of Bas Mitzvah. For in some places those who "scrupulously observe" the Bas Mitzvah ceremony burst into song, and unfortunately, in present times, this is done with the participation of women - mainly the Bas Mitzvah and her friends. In this matter compromise is impossible, for the Shulchan Aruch explicitly forbids this.

Excerpt from Igros Kodesh, Vol. XVII, p. 238