Israel's official calendar is the Hebrew one. According to Jewish counting,Read it fast, before it vanishes behind the paywall!
this Wednesday we will be starting Year 5775, that is - the supposed 5775th year since the world was created on Saturday night, October 6, 3761 BCE.
This reckoning was instituted by Maimonides in the 12th century, in the stead of the previous system Jews had used before, which counted from the destruction of the Temple in 70 CE.
Moving onto today: Israel's official calendar is the Hebrew one. Under law, official Israeli documents must have the Hebrew date on them. Moreover, holidays in Israel are determined according to the Jewish calendar, not the Gregorian one. Thus a given fest – say, Rosh Hashanah – will happen on the same date each year according to Jewish reckoning, but on a different day each year according to the Gregorian calendar. That is because the Gregorian and Jewish calendars don't coincide.
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There's lots of historical information here, including a discussion of the Gezer Calendar. But, surprisingly, there is no reference to the solar calendar used by the Qumran sectarians, the book of 1 Enoch and the book of Jubilees, on which more here and links.
Also, Philologos had a column some time ago on Jewish calendars. The link has rotted, but the excerpt gives an overview.
And, tangentially related, a while ago my colleague Grant Macaskill sent me this article from 2005, which tells how a physicist seems to have unwittingly reinvented the Enochic solar calendar, or at least something pretty close to it, and was trying to persuade people to adopt it: Novel calendar system creates regular dates (Maggie McKee, New Scientist).
A US physicist is lobbying for people to adopt his novel calendar in which every date falls on the same day of the week each year.Dr. Henry adds an intercalated week ("Newton Week") every five or six years. It isn't clear exactly how the Enochic Jews and Qumran sectarians dealt with the slight discrepancy between the 364-day solar calendar and the actual 365-and-a-quarter-day solar year. Like the Enochic calendar, his calendar has the advantage of keeping all the holidays on the same day of the week every year. This was more important for the Jewish calendar, since if holidays did not have a stable spot in the week (as they don't in the modern Jewish calendar), when a major holiday falls on the sabbath it can cause halakhic complications.
The current calendar, which runs for 365 days, was instituted by Pope Gregory in 1582 to bring the length of the year in line with the seasons. But because the Earth actually orbits the Sun every 365.24 days, a 366-day "leap year" must be added every four years to account for the extra fraction of a day. In this Gregorian system, a given date (such as New Year's Day) falls on different days of the week in different years because 365 is not evenly divisible by seven.
That means new calendars must be printed every year, and the dates for recurring events constantly recalculated. "For many years, I've had to make up a new schedule to tell my class when homework is due," says Dick Henry, a physicist at Johns Hopkins University in Baltimore, Maryland, US. "Here I am putting all this totally unnecessary work in and I decided I better do something about it."
So Henry designed a calendar that uses 364 days, which breaks down evenly into 52 weeks. In his so called "Calendar-and-Time" (C&T) plan, each month contains 30 or 31 days. He decided on each month's length by forbidding the new calendar to differ from the old one by more than five days and by setting Christmas Day, 25 December, to always fall on a Sunday.
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