According to some, the first documented Bar Mitzvah celebration is referred to in the Torah: "And the child [Isaac] grew and was weaned, and Abraham made a great feast on the day that Isaac was weaned" (Genesis 21:8). According to one opinion expressed in the Midrash, this was the day that Isaac turned thirteen; the day when he was "weaned" from his childish nature, and assumed the responsibilities of a Jewish adult. In Jewish literature, this verse is often used as a source for the celebration made in honor of a boy's acceptance of the mitzvot at age thirteen.
The Zohar1 relates how on the day of his son's Bar Mitzvah, Rabbi Shimon bar Yochai made a feast which was akin to a wedding celebration.
My research suggests that the celebratory Bar Mitzvah feast became a unanimously accepted Jewish custom some four hundred years ago.
As for the cause of the celebration, this is the day when a Jewish person is given the obligation and resulting privilege of observing G‑d's commandments. One would be hard-pressed to think of a more joyous occasion to celebrate together with friends and family!
Rabbi Shlomo Luria, noted 16th century Polish scholar, rules that the Bar Mitzvah feast is a seudat mitzvah, a "mitzvah repast," which means that participating in this meal is actually a mitzvah.
Does anyone know when a boy in Biblical times brought his own sacrifice to the temple? Was this connected at all to his "coming of age," or to being head of his own household? Some of my students asked me this, and I have no idea! From other reading I'm doing, it seems that boys were not restricted from religious participation in ancient times; it was only not required of them.
The concept of Bar Mitzvah, which literally means "Son of the Commandment," marks the time when a Jewish boy becomes religiously responsible for his actions. While there was no specific sacrifice required for a Bar Mitzvah in the Temple era, there were several occasions when a boy might bring his own sacrifice. Even younger boys were obligated to attend the pilgrimage festivals in Jerusalem. In particular, the Passover sacrifice was a communal obligation, and a boy who had reached the age of religious majority would participate in this ritual. As a newly recognized adult member of the community, a Bar Mitzvah boy might choose to bring voluntary offerings to express his devotion or gratitude.