Yes, simple First Grade arithmetic, a three day Jewish Holiday in the Land of one day holy days except for the traditional two day Rosh Hashannah. Over the four wonderful decades we've enjoyed as Israelis that long holy days weekend has been pretty rare, but things are changing. So save your menus and cooking tips from this year's three day holiday marathon, because we'll be doing it almost every year for the next few.
Are you wondering why? Well, here's Sabba Hillel's clear explanation:
The Jewish year has three possibilities for the number of days in a regular or leap year. The reason is that the lunar month is approximately 29.5 (twenty nine and a half) days long. If this was exact, then just alternating 29 and 30 day months would be correct. However, the exact average cycle is 29 days 12 hours 793 "parts" in length. A "part" is one in 1080 of an hour. The 793 "parts" converts to 44 minutes and 1 "part" as can be seen by looking at a chart of the molad announcements for the year. As a result, there are almost 15 minutes more than 29.5 days. This is handled by having the months of Cheshvan and Kislev be either 29 or 30 days and having the three possibilities of 29 and 29, 29 and 30, or 30 and 30.
Rosh Hashannah (first day) can never occur on Sunday, Wednesday, or Friday so that Yom Kippur cannot occur on Friday or Sunday and Hoshannah Rabbah can never occur on Shabbos.
This makes the regular year have 353, 354, and 355 days ("adding 3, 4, or 5 days to the day of the week of Rosh Hashannah), while leap years have 383, 384, and 385 days ("adding" 5, 6, and 0). The Jewish year uses a 19 year cycle and the leap years can be shown by taking the year number as modulus 19. This year (5771) is year 14 of the cycle (year 19 of the cycle has modulus 0). The five years involved are thus 14, 15, 16, 17, 18 Year "17" is the next leap year. This year (a leap year) both Cheshvan and Kislev are 30 days so that next Rosh Hashannah (5772) will be also be on Thursday and Friday. The following year will have Cheshvan 29 days and Kislev 30 days (30 Kislev 5772 is 26 December 2011). This brings the following Rosh Hashannah (5773) to Monday and Tuesday. .
5773 will have 29 days in Cheshvan and Kislev which brings Rosh Hashannah back to Thursday again (from the modulus calculation above.) for 5774.
5774 is year 17 of the 19 year cycle and is again a leap year and again will have both Cheshvan and Kislev set to 30 days. This means that the following (regular) year of 5775 will again start on Thursday.
Since 5775 is not a leap year, Rosh Hashannah of 5776 will occur on Monday and since it is again a leap year (modulus number 0) with both Cheshvan and Kislev 30 days, the following year (5777) is again on Monday.