The Jewish Calendar and Holidays (incl. Sabbath): Other Holidays and Fasts: Shavuos on Sunday:
According to the Torah (Lev. 23:15), the omer is brought on "the
day after the Sabbath", and we then count seven weeks and celebrate
Shavuos. Why doesn't Shavuos always fall on a Sunday?
"The Sabbath" in Lev.23:15 means "The holiday", i.e. the first day of
Passover. The omer offering was brought on the second day of Passover,
i.e. on the 16th of Nisan, which does not always fall on the same day
of the week, and Shavuos falls seven weeks later. Holidays are called
"Sabbaths" because most work is forbidden on holidays; e.g., see
Lev.23:7 about the prohibition of work on the first day of Passover.
The weekly Sabbath is called a "Sabbath of Sabbaths" because even more
types of work are forbidden; see Lev.23:3.
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