Calendar dating for previous years
Holidays are celebrated on the same day of the Jewish calendar every year, but the Jewish year is not the same length as a solar year on the civil calendar used by most of the western world, so the date shifts on the civil calendar.
The Jewish calendar is based on three astronomical phenomena: the rotation of the Earth about its axis (a day); the revolution of the moon about the Earth (a month); and the revolution of the Earth about the sun (a year).
The months drift around the seasons on such a calendar: on a 12-month lunar calendar, the month of Nissan, which is supposed to occur in the Spring, would occur 11 days earlier in the season each year, eventually occurring in the Winter, the Fall, the Summer, and then the Spring again.
On a 13-month lunar calendar, the same thing would happen in the other direction, and faster.
A few years ago, I was in a synagogue, and I overheard one man ask another, "When is Chanukkah this year?
" The other man smiled slyly and replied, "Same as always: the 25th of Kislev." This humorous comment makes an important point: the date of Jewish holidays does not change from year to year.
The extra month is inserted before the regular month of Adar (known in such years as Adar II, Adar Sheini or Adar Beit).
If you are musically inclined, you may find it helpful to remember this pattern of leap years by reference to the major scale: for each whole step there are two regular years and a leap year; for each half-step there is one regular year and a leap year.
This is easier to understand when you examine the keyboard illustration below and see how it relates to the leap years above.
The Earth revolves around the sun in about 365¼ days, that is, about 12.4 lunar months.
The civil calendar used by most of the world has abandoned any correlation between the moon cycles and the month, arbitrarily setting the length of months to 28, 30 or 31 days.