Rosh Chodesh, Jewish Leap Years, Adar II, and Joy


Rosh Chodesh, Jewish Leap Years, Adar II, and Joy


Search our Archives:

Opinion & Society

The Jewish Leap Year – Adar II

By Nachum Mohl

In the solar calendar there is a concept of a leap year – February will have twenty-nine days instead of the usual twenty-eight. This comes once every four years. The Jewish calendar has a concept of a leap year too, but it is radically different than the common calendar leap year – we add another month!

To understand how the Jewish leap year works, we must first define a year. Simply speaking, a year passes by when the sun's position in the sky returns to the exact position that it was in relation to the season. Each day when we go outdoors, the sun is constantly shifting in the sky, moving up and down in the heavens, shifting its position from where it rises in the east and where it sets in the west, and how high it rises in the south. This solar cycle takes 365 days and a little under six hours.

But the Torah has fixed the Jewish month based on the moon – not on the sun. The moon begins at the beginning of each Jewish month as a thin crescent and gradually grows fuller each night until it is perfectly full and round. This marks the middle of the Jewish month. Then the moon begins it gradual reduction until it disappears only to reappear again at the beginning of the new month. When the moon first appears as a narrow crescent it is called the New Moon or the beginning of a new month, in Hebrew: Rosh Chodesh.

It takes the moon a little over 29 ½ days for the moon to complete its monthly cycle. Since we cannot have part of a day belonging to one month and part of the day belonging to another, the calendar is arranged so that some days are 29 days long and some days are 30 days long. A month is never more than 30 days nor less than 29 days.

This explains why we sometimes two days Rosh Chodesh (the beginning of the month) and some times only one day Rosh Chodesh. When we have one day Rosh Chodesh it means that the out going month had 29 days; when there is two days Rosh Chodesh it means that the first day of Rosh Chodesh is the last day of the out going month and the second day is the first day of the incoming month. The only except to this rule is the month of Tishre when the Rosh Chodesh is Rosh Hashanah; then the first two days of Rosh Chodesh are Rosh Hashanah which are the first and second days of the New Year.

Now although the months go according to the moon's cycle, the year must be reckoned in consideration to the sun's cycle. The reason is that Torah was particular that the holiday of Passover should fall in the spring. The moon's cycle has no relation to the seasons, but the sun's cycle is related. In the summer the sun is high in the noonday sky, in the winter it is low. During spring and fall the sun's height as measured by the noon day position is in an intermediate position.

Since the holiday of Passover must be observed in the spring, we must reckon the counting of the months in a manner that the month of Nisan (in which Passover comes) is always in the spring. Now the four seasons take up 365 ¼ days, yet the moon's cycle is only 29 ½ days. If we multiply 29 ½ days by twelve months we get 354 days which leaves us some 11 days short of a solar year. That means that every year the months move back about one third of a month and in nine years the Jewish holidays would fall behind the solar year and seasons by about three months. If we allowed this to happen, Passover would be in the summer and then in a few more years in the winter! Yet the Torah explicitly stated that Passover should be celebrated in the spring.

Therefore to keep the festivals on track an extra month is added once in about every three years when the 11 day difference grows into a month. This extra month is added after the month of Sh'vat and before the month of Adar that has in it Purim. We call this month Adar I and the Adar that has in it Purim, we call Adar II. In this manner Nissan, the month that has Passover, is pushed back into its rightful place in the sequence of the seasons. Once Nissan is in its proper place, then all the subsequent months and their festivals, Shavuot and Succot, fall into their proper places.

During the time of the Temples, the months were declared according to visual testimony in the Jewish Supreme court, the Sanhedrin. Since the destruction of the Temple, and the demise of the Sanhedrin, we rely on the fixed calendar for all of our months and festivals. The sages who worked out this calendar were wise in astronomy and mathematics and fixed it for all generations. The following is their method of calculation:

A leap year cycle is a nineteen year cycle. During this period of time there are seven leap years: the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years in the cycle are the leap years. We can figure out if the year is a leap year by dividing the present Hebrew year by 19. If the remainder number is one of the above numbers or zero (in the place of 19) then it is a leap year. For example, this year is 5768; if we divide it by 19, we get 305 with a remainder of 11, which tells us that this is the 11th year in the 19 year cycle. The next leap year will be in the 14th year (in three more years).

If a person was born in Adar, in which Adar does he celebrate his birthday? The usual custom is to celebrate the birthday in the same Adar in which Purim falls, meaning Adar II. However if he was born in Adar I in a leap year, then he would celebrate his birthday in Adar I. Conversely, if someone was born in Adar II, he celebrates his birthday in regular years in the only Adar that comes, regular Adar. This can present a small problem, if one person was born in the twentieth of Adar I and his friend was born in the next month on the third of Adar II, if their bar mitzvah is in a plain year (with only one Adar), the younger boy (born in Adar II) will celebrate his bar mitzvah on the third of Adar before his older friend (born in Adar I) on the third of Adar. Ah, such is the irony of the Jewish calendar!

During a leap year, the date in Adar I of Purim is not celebrated as Purim, however it has become a festive day in that certain prayers are not said, and the custom is to eat in a more festive manner. It is called Purim Katan, meaning the small Purim.

The rule is that we increase joy, never decrease, therefore a year with two Adars in it is considered a happy year.

* * * * *

For more Purim Articles, see our Purim Archives


from the February 2008 Edition of the Jewish Magazine

Please let us know if you see something unsavory on the Google Ads and we will have them removed. Email us with the offensive URL (