What is the origin of the festive Bar Mitzvah celebration?
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 Zohar 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.