Hot answers tagged calendar
http://kiddushhachodesh.com/ has many videos decently done.
Essentially the simplest answer is that, at the time of the Manna, they were told then "Today is Friday, gather double and save over because tomorrow is Shabbat and you won't find". (Beshalach). So with the Manna cycle it became "known" which day actually was Shabbat and once that happened the Jews have kept a record of it ever since and therefore in the ...
At the very least, from the parsha of מן in parshas Bishalach chapter 16 the days of Friday and Shabbos were clear. This was pre- Har Sinai.
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