LifeCycle Events: Bar Mitzvah: Fasting on One's 20th Birthday:
Is there a custom to fast on one's 20th birthday?
The book Hilchos ve-Halichos Bar Mitzvah, which has four pages on laws that apply at age 20, says nothing about fasting when one reaches that age. But it's clear that the reason for such a custom would be that at age 20, one becomes subject to punishments (oneshin). In the Talmud (Moed Katan 28a) we find that Rav Yosef made a celebration when he reached 60 because he was no longer subject to the punishment of "cutting off" (karess). If becoming exempt from an onesh calls for a celebration, it's logical that becoming
subject to oneshin might call for a fast.
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