As the Jewish holidays fluctuate from year to year, people often ask me why they move around so much. "Rabbi, Rosh Hashanah was early in 2004, but Pesach was late. Rosh Hashanah will be late in 2005, but Pesach will be early in 2006. Why? And how come Easter almost always falls near Passover, but this year they were several weeks apart? So here goes at explaining.

All holidays are calculated on the Jewish calendar, which is lunar, meaning that each month begins with the new moon. A lunar month is 29 days long, so some months are 29 days long, some are 30. Twelve lunar months total 354 days, which means a lunar year is 11 days shorter than the solar year (which is the basis of the Gregorian calendar we use every day). Without an accommodation for the difference, the holidays would continue to move back 11 days each year, and we'd be celebrating holidays in the wrong season. This is especially problematic for those holidays which mark planting and harvesting.

To solve the problem, almost two thousand years ago the rabbis created a system in which an extra month was added seven times every 19 years (sometimes every 2 years, sometimes every 3 years). So every 20th year, a holiday falls on the same date on the Gregorian calendar.

When it isn't a leap year, a holiday moves back 11 days (e.g. in 2001 Rosh Hashanah was on September 18; in 2002 it was on September 7). When it is a leap year, a holiday moves forward either 19 or 20 days, based on how many 29-day months and how many 30-day months fall in the year.

The one confusing factor is that Rosh Hashanah begins the Jewish ritual year but not the Jewish calendar year. The extra month is added right before Pesach which occurs in the first calendar month of Nisan. That means if Pesach is early, the following holidays occur early, if Pesach occurs late, the following holidays occur late. The year we are in is toward the end of a 19-year cycle, so the holidays are the latest they can possibly occur.

The reason Easter usually falls close to Pesach is that Easter too is calculated on the lunar calendar - it is the first Sunday after the first full moon on or after the spring equinox. Coincidentally the first full moon fell near the equinox this year, so Easter was a little early. Because Pesach falls as late as possible, the two holidays were farther apart than normal.