The reason for this is that the Jews have a lunar calendar, now modified in form but originally reckoned by the moon.In ancient times the appearance of the new crescent after sunset, following several moonless nights, marked the beginning of the first day of each new month.Obviously then, if the 7th day of a period ends at sunset, then all the days of the period must end at sunset.The Week Marked Off by the Sabbath.—The week was divinely marked out, even before the giving of the law, by the double portion of manna on the 6th day and the withholding of it on the 7th (Exodus 16).We find the answer to this puzzling situation, and to other problems, by a study of the origin and nature of the Jewish calendar as set forth in the Bible and other ancient records.The early Hebrew calendar as given in the Bible was admirably adapted to the needs of an ancient people who had no clocks, no printed calendars, and, as far as we know, no astronomy.It was the only element of the calendar enshrined in the Decalogue, for the Sabbath has a moral aspect that is not connected with mere dates and calendars.
If they had retained the method of adding a month periodically, as was done in Mesopotamia by the Babylonians and Assyrians, we have no record of it.
Shortly before the Exodus He instructed Moses that “this month shall be unto you the beginning of months: it shall be the first month of the year to you” (Exodus 12:2).
There was no systematic code of calendar rules, but the civil and ceremonial laws given through Moses contain incidental references to the elements of the calendar.
Those who have Jewish neighbors know that they celebrate their New Year’s Day, which they call Rosh Hashana, in the autumn.
If we ask a rabbi the date of Rosh Hashana, he will explain that it is the first of the Jewish month Tishri, but that it falls on different dates in our September or October in successive years, since it comes approximately at the new moon.