The Jewish Calendar and Holidays (incl. Sabbath): The Jewish Calendar: The Jewish Holidays:
Why don't the Jewish holidays occur on the same secular dates every year?
The Jewish year is usually 12 lunar months long, but 12 lunar
months add up to only 354 days. To keep the Jewish year in
step with the 365-day solar year, some Jewish years (7 out of
19) are 13 lunar months long. Thus the 12-month years are 11
days shorter than a solar year, and the 13-month years are
about 19 days longer. As a result, the Jewish calendar shifts
back and forth relative to the secular calendar, but it comes
out even by the end of the 19-year cycle.
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