I'm looking a the calendar and I see that tomorrow, September 22, is the first day of Autumn. I also see that September 21 is Rosh Hashanah. What exactly is that?

In Hebrew rosh has many meanings, including “head” or “first” or “start,” and shanah means “year,” with ha simply meaning “the.” In combination, the name of the holiday translates as “head of the year.” Rosh Hashanah is one of the most important holidays in Judaism.

Rosh Hashanah began at sundown  on Wednesday, September 20. The holiday ends with nightfall Friday, September 22. 

According to Newsweek.com, the celebration marks the start of the year 5778 on the Jewish calendar and marks the number of years since the world was created, at least according to the story recounted in Genesis, the first book of the Hebrew Bible (what Christians would call the Old Testament). The day itself is the anniversary of the day the first man and woman, Adam and Eve, were created.

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