Relative to our secular calendar, this year the Jewish High Holidays are late. As recently as 2013, Rosh Hashanah eve occurred on September 4th. But this year, Rosh Hashanah eve falls on Sunday, October 2nd. Why doesn't Rosh Hashanah fall on the same date on our secular calendar every year?
Our secular calendar is a solar calendar. It is based on the amount of time it takes the earth to revolve around the sun, which is 365 and 1/4 days. So there are 365 days in our secular year except every fourth year - leap year - when February has an extra day - 29 days - and, thus, there are 366 days in the year. The position of the earth relative to the sun as it revolves around the sun determines the season of the year on different parts of the earth.
Unlike our secular calendar, the Jewish calendar is a lunar calendar. Every month begins when a new moon (little sliver) appears in the sky. A new moon appears every 29 1/2 days. This means that in a twelve month year, half the months will have 29 days and the other half will have 30 days, leaving us with a year of 354 days. Since the seasons of the year are determined by the earth's position in relation to the sun as it revolves around the sun, the lunar calendar, if left unadjusted, over a period of years, would cause Jewish holidays to shift to different seasons of the year. Thus, over a period of years, a holiday like Passover, which is a springtime holiday, would occur in other seasons of the year.
To insure that holidays remain in the same season of the year, the rabbis adjusted the Jewish lunar calendar to conform to the secular solar calendar in the long run. So seven out of every nineteen years on the Jewish lunar calendar is a Jewish leap year. But in a Jewish leap year, instead of adding an extra day, an additional month is added. Instead of twelve months in a Jewish leap year, there are thirteen months. The holiday of Purim occurs in the Jewish month of Adar. In a Jewish leap year, there are two months of Adar (Adar I and Adar II) and Purim is celebrated in Adar II.
This may all be a bit confusing, but, just remember; in the long run it all works out.
Rabbi David Weissman