The Torah prescribes the observance of festivals on very specific dates, but does not explain how the calendar must be reckoned: Is it lunar? Is it solar? Does it follow some other scheme? And why is the Torah silent on this?There's no particular reason that either the Bible or the Torah have to have a single answer to this question. There were competing Jewish calendars in the Second Temple Period (as the essay discusses).
Some past posts on ancient Jewish calendars are here and follow the links. And I have collected posts on the somewhat related question of the chronology of the Gospel Passion narratives here.
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