Celebrating a Bar Mitzvah has a source in Torah, whereas celebrating a Bas Mitzvah does not have an official Torah source. Though the concept of a girl becoming Bas Mitzvah at the age of twelve is sourced in Torah, the idea of making a special Bas Mitzvah celebration was introduced by foreign sources, people outside the parameters of traditional Yiddishkeit who attempted to emulate other religions and ensure that girls had the same celebration as boys. Although it strong is strong appropriate to celebrate a Bas Mitzvah which is the time when a girl becomes mechuyav, obligated, in mitzvos, nevertheless it should not be done in a way similar to how it is celebrated in foreign circles by the mischadshim, those who invent new ideas which are not traditional Torah ideas. A Bas Mitzvah celebration should be done in a modest way, such as part of a Mesibos Shabbos, a Melave Malka, or similar to a yom holedes celebration (a modest and spiritually meaningful birthday party). The parents should buy the girl a beautiful and meaningful present, and everything should be done in the most tzniyus, modest, manner. Most important is that the girl understands that at this stage of her life she is becoming obligated in the chiyuv of mitzvos, and takes on the responsibilities of being a Jewish woman with joy and gladness of heart.