Reference for Chagigah 33:4

אין כהן גדול מתלבש בכליו ומותרין בהספד ובתענית שלא לקיים דברי האומרין עצרת אחר השבת:

The High Priest does not wear his vestments, and we may eulogize and fast [on the day of slaughtering that falls out on Sunday], so as to uphold the words of [the Sadducees] who say, "Shavuot is after Shabbat."

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