THE FESTIVAL OF SHAVUOT begins tonight at sundown. (Best wishes to all those celebrating.) But if the Sadducees had had their way, it would have started last night. (Apparently this is in the Talmud, but as usual no reference is given.)

UPDATE: Manuscript Boy e-mails the following:

Menahot 65a, which is a quote from the scholion of Meggilat Ta'anit. See Vered Noam's new edition of the Megillah, p. 59 (Nissan 8 until the end of the Moed).

Carla Sulzbach sends the same reference. That's the Babylonian Talmud.