Rosh HaShanah (literally Head of the Year in Hebrew) is celebrated on the first and second days of the Hebrew month of Tishrei, 163 days after the first day of Passover. The Hebrew calendar is composed of 12 lunar months (13 in a leap year, when the month of Adar is doubled). While there are actually four different “New Year’s Days” established in this calendar for different purposes (mostly agricultural), when we speak of the Jewish New Year, we mean Rosh HaShanah. It was ordained in the Torah as a celebration on the first day of the seventh month, but was selected by the rabbis of the Mishnah to be the normative day for calculating years.
Since the common or Gregorian calendar is based on the sun rather than on the moon, the date of Rosh HaShanah varies from year to year. Due to the complicated relationship between the two calendars, it comes earlier and earlier every year – until a Jewish leap year, with 13 months rather than 12, pushes it later again. Also, since the fourth century C.E., the Hebrew calendar has been subtly manipulated to ensure that the first day of Rosh HaShanah never falls on a Sunday, Wednesday or Friday.
The following table shows how the dates move back and forth.
Gregorian Year First Day of Rosh HaShanah
2012 September 17
2013 September 5
2014 September 25
2015 September 14
2016 October 3
2017 September 21
2018 September 10
2019 September 30
2020 September 19
Note that unlike Gregorian leap years, Jewish leap years do not come at regular four-year intervals. They are calculated in a very complicated fashion to occur seven times in 19 years – dividing one prime number by another is very tricky.
Another important thing to know about Jewish dates in general, and that of Rosh HaShanah in particular, is that they begin at sundown the night before. Unlike Gregorian dates, which tick over at midnight, a Jewish day begins at sundown. A relic of this ancient way of counting time can be found in the Christian tradition of celebrating Christmas Eve and Halloween (the eve of All Hallows Day). In Hebrew, we use the term “erev” (evening) to denote the day before a holiday – so if the first day of Rosh HaShanah is September 17th, the holiday actually begins at sundown on September 16th.