A friend told me that due to how the Jewish and Gregorian calendars are set, one’s Jewish and secular birthdays coincide every 19 years. But I just checked your online calendar, and it seems that my 19th birthdays will not coincide. Did I somehow miscalculate my Jewish birthday or is my friend wrong?

This is indeed conventional wisdom, and the 19th birthdays indeed coincide for many people, but not for most. To understand this, a general understanding of the Jewish calendar and, more specifically, where the number 19 comes, from is in order.

## The Lunar and Solar years

A year based on the solar calendar is approx 365.24 days (which is then broken into 12 months).

The months of the Jewish calendar follow the lunar orbit, which is approximately 29.5 days. Twelve such months make a year.

An average Jewish year has twelve months: six 29-day months, and six 30-day months, for a total of 354 days (although due to variations in the Jewish calendar, the year could also be 353 or 355 days).

Note that the lunar year is thus a bit more than 11 days shorter than the solar cycle. This presents a problem, since it is a mitzvah to make sure the Jewish holidays fall out in the proper seasons.

Passover, for example, should always be in the spring.1 Left unchecked, Passover would move back 11 days every year and end up sometime in the winter!

To keep the holidays in sync, we add a (30-day) month every few years, creating a 13-month year. Such a year is called a shanah meuberet ("pregnant year") in Hebrew, or “leap year” in English.

These leap years run in 19-year cycles, with each cycle containing seven leap years. (The 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years of the cycle are Jewish leap years.)

On average, at the end of a 19-year cycle, the Jewish calendar would have 6939.69 days.2 And 19 solar years translate into about 6939.61 days.3

Since both cycles are more or less in sync in terms of the number of days in a 19-year cycle, it makes sense that often the Jewish and civil birthdays coincide every 19 years.

Yet, due to a number of reasons, the two birthdays are more often not in sync.

## When Does the Day Start?

One very basic reason why the two birthdays may not be in sync is that in truth they were off from the very first day you were born!

A Jewish date starts at nightfall. A secular day, however, begins at midnight. Let’s say May 5th coincides with the 4th of Iyar, nightfall is at 8:00 pm, and you were born after nightfall. So your Jewish birthday would be the 5th of Iyar, but your civil birthday would still be on the 5th of May.

Thus, you would have started the 19-year cycle a day off.

## Jewish and Civil Years Are Not Uniform

However, even if you were born during the day, there is yet another reason why the two birthdays would, more often than not, not coincide every 19 years.

Although we said that a cycle of 19 years on the Jewish calendar is 6939.69 days, that is not entirely accurate.

Due to the lunar cycle being 29.5 days, the months of Marcheshvan and Kislev on the Jewish calendar can have either 29 or 30 days. These two variable months are not dependent upon the 19-year cycle. Due to these variations, a 19-year cycle can be 6,938, 6,939, 6,940, 6,941 or 6,942 days long.

Additionally, although most years February has 28 days (due to the solar calendar having 365.25 days), every four years there is a civil leap year, and February has 29 days. Furthermore, to compensate for the fact that a solar year is slightly less than 365 days by about 11 minutes, the leap year is omitted three times every four hundred years.

In other words, on the Gregorian calendar, a century year cannot be a leap year unless it is divisible by 400. Thus 1700, 1800 and 1900 were not leap years, but 1600 and 2000 were leap years.

Thus, the number of days in 19 civil years depends upon where in the civil leap-year cycle the 19 years begin, and the 19 years can have either 6,938 days (if it includes one of the 100th years which is not a leap year) 6,939 days (if there are four leap years) or 6,940 days (if there are five leap years).

So if the birthdays ever coincide, it is most common that it will happen every 19 years, but, for the reasons stated above, that is often not the case either.

On the topic of Jewish birthdays, it should be noted that in 1988 the Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, launched a "Jewish Birthday Campaign." He enjoined us to utilize this special day as a time of introspection and recommitment to the mission that G‑d entrusted to us—bettering and sanctifying ourselves and the world around us.

For more on your Jewish birthday, including how to calculate when it is, visit The Jewish Birthday mini-site.