cross-posted from: https://hexbear.net/post/603317

Link to a more exhaustive history of the Jewish Calendar:

From the Jewish Encyclopedia (1906)

Happy Rosh Hashana 5784 to all my Hexbears who celebrate!

The Jewish calendar is pretty wacky! It incorporates both Solar and Lunar cycles and the new year actually takes place in month #7. Though the Canaanite Gezer Calendar begins the year in the early Autumn, but does not include the names of the months. The months are of Babylonian origin, named:

1. Nissan
2. Iyar
3. Sivan
4. Tammuz
5. Av
6. Elul
7. Tishrei
8. Heshvan
9. Kislev
10. Tevet
11. Shvat
12. Adar
13. Adar 2 (sometimes)

You'll notice an occasional month #13. Since the months are lunar, they don't perfectly sync up to a full revolution of the the earth around the sun. The extra month is necessary to keep Passover in the spring (month of Nissan) After much discussion included consultation of Pagan astronomers (this was allowed due to their accuracy) during the 300s C.E., a regular system was developed where 235 lunar months match up to 19 solar years. If the year, when divided by 19 has a remainder of 0,3,6,8,11,14, or 17 it will have a "leap month" added, or else it will have 12 lunar months.