The Jewish Calendar and Holidays (incl. Sabbath): The Jewish Calendar: How is the Jewish Calendar Fixed?:
How is the Jewish calendar fixed?
Fixing the calendar is the responsibility of the Sanhedrin (the highest Jewish court); today we use rules that the Sanhedrin formulated about 1500 years ago. Since the calendar underlies all the commandments that have to be observed at fixed times of the month or year, Maimonides' Mishneh Torah treats the calendar in the third book, Zemanim.
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