Bloodletting on the New Moon and on the second day of the month causes weakness; bloodletting on the third day of the month leads to danger. Bloodletting on the eve of a Festival causes weakness; bloodletting on the eve of the festival of Shavuot leads to danger. And the Sages issued a decree prohibiting bloodletting on the eve of every Festival due to the festival of Shavuot. On Shavuot, an evil spirit named Tibbuaḥ, from the Hebrew word meaning slaughter, emerges, as had the Jewish people not accepted the Torah on the festival of Shavuot, Tibbuaḥ would have slaughtered their flesh and their blood. Consequently, it remains a dangerous day.
Shabbos139b
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