I found this helpful explanation of the arrangement of the Hebrew calendar here. Since this is something I always seem to have trouble explaining, I thought I'd pass it along.
Our scholars used a calendar cycle of 19 years consisting of 12 years of 12 lunar months each and 7 years of 13 lunar months each for a total of 235 lunar months. The Hebrew name for this cycle is mahzor qatan.At some point in the history of the calendar, the beginning of the very first period of 19 years was determined, and years 3, 6, 8, 11, 14, 17, and 19 of the 19 year cycle were declared to be leap years of 13 months each.
This distribution of the leap years ensured that all Hebrew years in the 19 year cycle would begin, arithmetically at least, less than one lunar month after the start of their corresponding solar year in that cycle.The distribution is easily remembered by the mnemonic GUChADZaT which stands for the Hebrew letters gimel-vov-het aleph-daled-zayyen-tet.
A given Hebrew year is a leap year whenever its value divided by 19 leaves a remainder that is either 0, 3, 6, 8, 11, 14, or 17. In pre-zero arithmetic the number 19 corresponded to the zero remainder.
For example, the year 5757H (1996g/1997g) was a Hebrew leap year because after division by 19 the remainder is 0. That, by the way, also made year 5757H (1996g/1997g) the last year of the 303rd 19 year cycle.
["H" designating Hebrew calendar years and "g" designating Gregorian calendar years]In a Hebrew leap year a 30 day month is added to the year. This month is today known as the month of Adar I, or Adar Alef, or Adar Rishon, and is inserted immediately after the Hebrew month of Shevat. In our times, the insertion tends to take place in the February/March period of the Gregorian calendar year.
Presently, Hebrew leap years can begin no earlier than September 5 and no later than September 16, while Hebrew common years can begin no earlier than September 16 and no later than October 5.
We're currently in the middle of a leap year. Next year, the year 5766, is a common year. It begins on October 4th, which is just about as late as can be. And that explains why all of the Jewish holidays, starting with Passover this year and extending through next Purim, are going to be really, really late, as well.
Shabbat Shalom.
