Bmitzvah.org: The Bar Mitzvah & Bat Mitzvah (R)evolution continues
Reb Nachman of Breslov taught: Mitzvah gedolah liheeyote b’simcha tamid - "it is a great mitzvah to always be at a simcha." Simcha can be translated as a state of happiness or a happy occasion. In Genesis 21:8 Abraham threw a feast to celebrate Isaac’s weaning, and there is a midrash (rabbinic interpretive tale), in Bereshit Rabbah 53:10. which says this was when Isaac was thirteen years of age.
