How does the spring equinox relate to the timing of Passover?

The Jewish calendar normally consists of twelve lunar months. A lunar month -- from the moment when the crescent new-moon appears until it disappears once again -- is roughly 29.5 days. Twelve lunar months equal 354 days; eleven days less than the solar year. Therefore, if we were to maintain a strictly twelve-month lunar calendar, we would lose eleven days each year. This would result in holidays which would constantly be fluctuating in relation to the seasons, which are dictated by the solar cycle. We would have summer Chanukahs and snowy Shavuots.

Therefore the Torah commands us,1 "Guard the month of spring, and make [then] the Passover offering." this is a directive to the Sanhedrin (Rabbinic Supreme Court) to constantly adjust the calendar to ensure that Nissan, the month of the holiday of Passover, always falls during the spring season. This is accomplished through thirteen-month "leap years" which were added to the calendar approximately once every three years. During these years, a second month of Adar was added to the calendar.

While the Sanhedrin presided in Jerusalem, there was no set calendar. They would evaluate every year to determine whether it should be declared a leap year.

Several factors were considered in the course of their deliberations. The primary factor, which overrode all others, was the spring equinox. If the spring equinox would fall later than the first half of Nissan (i.e. on the 16th or later), then the year was automatically declared to be a leap year.

However, it wasn't enough for Passover to fall after the equinox, when it was "officially" spring; spring-like conditions needed to be evidenced. If in the land of Israel the barley2 had not yet ripened, and the trees were not yet blossoming with seasonal fruit -- that, too, was sufficient reason to delay Nissan through adding a second month of Adar. Spring should be felt; it should be bright and green.

There were also several non-season-related factors which the Sanhedrin considered. For example, if the roads or bridges were in disrepair due to the winter rainy season, impeding the ability of the pilgrims to travel to Jerusalem for Passover.

In the 4th century CE, the sage Hillel II foresaw the disbandment of the Sanhedrin, and understood that we would no longer be able to follow a Sanhedrin-based calendar. So Hillel and his rabbinical court established the perpetual calendar which is followed today. This calendar is comprised of nineteen year cycles, each cycle containing seven leap years. This calendar will remain in effect until Moshiach will come and reestablish the Sanhedrin.

Wishing you a happy, healthy and kosher Passover!

Yours truly,

Rabbi Menachem Posner