1. Declaring a Leap Year Is Part of the First Mitzvah

In Exodus 12 G‑d commanded us to observe Passover in the spring. The Hebrew leap year ensures that the Jewish calendar remains true to the solar cycle so that the holidays are celebrated at the right time.

2. A Month Is Added

Unlike the Gregorian (and Julian) leap year, in which an extra day is added, the Jewish leap year has an entire extra month.

3. Called an "Enlarged Year"

On the secular calendar, the date drifts by one day per year. So if January 1 is on Sunday this year, it will be on Monday next year, and so on. In the event of a leap year, the extra day will cause it to “leap” from Sunday to Tuesday, hence the name “leap year.” This, of course, does not apply to the additional month added to the Jewish calendar. The Jewish leap year is known as a shanah me’uberet, a “pregnant year,” or perhaps more properly an “enlarged year,” since it is temporarily larger than usual.

4. The Secret

The various calculations that go into determining the length of each year are known as sod ha’ibur. Sod can be translated as either “secret” or “council.”

5. It Was Done By the Central Court

Declaring a leap year was the domain of the highest rabbinical court—the Sanhedrin. According to Rabbi Meir, it was conducted by a panel of three judges. Rabbi Shimon ben Gamliel, however, taught that three sages would begin the conversation, two more would join for the deliberation, and another two were added for the final declaration.

6. The Sages Were Expert Astronomers

The primary factor, which overrode all others, was the spring equinox. If it were to fall later than the first half of Nissan (i.e., on the 16th or later), then the year was automatically declared a leap year.

Spring-like conditions also needed to be evidenced. If the barley in Israel had not yet ripened, and the trees were not yet blossoming with seasonal fruit—that, too, was sufficient reason to delay Nissan by adding a second month of Adar.

7. There Were Other Factors Too

The Sanhedrin also considered several non-season-related factors. If the roads or bridges were in disrepair due to the winter rainy season, for example, that would impede the ability of the pilgrims to travel to Jerusalem for Passover. Declaring a leap year would give crews time to get everything in order.

8. It is Now Fixed to Happen 7 out of 19 Years

In the 4th century CE, the sage Hillel II foresaw that the central courts would soon crumble and ad hoc declaration of leap years would no longer be viable. He and his rabbinical court established the perpetual calendar which we follow today, with nineteen-year cycles, each cycle comprising seven leap years.

9. It Can be 385, 384, or 383 Days Long

The longest possible year is 385 days. At times, however, a leap year can be only 383 days. This is because the months of Cheshvan and Kislev can each have either 29 or 30 days.

10. Contracts Can Be Complicated

If a rental contract specified a price per year, and then a leap year was declared, the 13th month was included in the set rent. If the contract specified the price per month, however, the 13th month must be paid for separately. In the event that the contract listed both (100 per month, 1,200 per year, for example), the renter is at a disadvantage and must pay for the 13th month.

11. The Added Month is Adar I

How does a 12-month calendar suddenly become 13 months long? The last month of the year, Adar, becomes Adar I and Adar II. It is questionable which Adr is considered the Adar. All agree, however, that Purim (held annually on Adar 14 and 15) is celebrated in Adar II, so that is just a short month away from Passover.

12. 60 Days of Joy

Our sages say, “When Adar enters, we increase in joy.” Increasing in joy every day, an additional Adar means that there is so much more joy. That’s something worth celebrating!

13. We Have Tons More For You to Read About the Jewish Leap Year

That was all just a taste. Want to learn more about the leap year, its significance, and application? Here are some articles where you can explore the Jewish leap year in depth: