My understanding is that the Jewish calendar is based on a 360 day year. How then, do they keep the Festivals at the same time of year each year? Spring and Fall, etc. Among other problems, they would lose 5 days a year, each year. Every 73 years, or so, they would lose a full year. And the festivals would move around throughout the seasons – Hanukkah in summer, etc. How does that work?
According to some scholars, the 360 day calendar you refer to is really God’s calendar. They claim it’s the creation’s original calendar. Several major prophetic passages can only be reconciled by using this calendar. For example the duration of the Great Tribulation is variously given as 3 1/2 years (Rev. 12:14), or 42 months (Rev. 11:2) or 1260 days (Rev. 12:6). These three measurements are only the same on a 360 day calendar. I think adhering to the 360 day calendar is God’s way of keeping prophecies clear and specific no matter what mankind has done to the world’s various calendars.
The current Jewish calendar is based on lunar cycles making it about 11 days shorter than our solar based calendar (354 days vs. 365 days). To maintain a general alignment between the two, the Jewish people periodically add a 13th month to their calendar. This happens 7 times every 19 years, and keeps the Jewish festivals in the same general seasons relative to ours. Even so, the dates of their festivals can vary on our calendar by as much as a month or so from year to year.