Hebrew Calendar

The Bible says the Jews left Egypt at midnight by the light of the full moon, the moon of the spring equinox, on the fourteenth night of the month Nisan, about thirty-two hundred years ago. This, then, is the night for observing Pesakh, the Passover.
An immediate timing difficulty arises.
The Jewish year, like the Mohammedan, has twelve moon months of twenty-nine or thirty days.
The year of the sun, which governs the seasons, is about eleven days longer.
A moon calendar drifts backward at the rate of about a month every three years. Mohammedans successively observe Ramadan in winter, fall, summer, and spring.
But the Mosaic Law specifies that Passover is a spring holiday; the freedom feast must come in blossom time.
The old Jewish solution of this problem was a leap month every few years, proclaimed by the Sanhedrin.
When the dispersion destroyed the nation, and communication between the centers of learning in exile began to break down, the Rabbis worked out a perpetual calendar on a nineteen-year cycle, with seven leap months so arranged as to keep Passover for every at the equinox.
This calendar has the respect of modern astronomers.
In nearly two thousand years Passover has not drifted out of the springtime, and in foreseeable future it will not.
In ancient days each new month was proclaimed by the central court in Jerusalem when the crescent moon appeared.
The only doubt was whether it would show up twenty-nine or thirty days after the last new moon.
As soon as the month was announced, runners went out all over the Mideast to advise the Jewish communities of the date of their holy days.
Those settlements that were more than fourteen days’ journey from Jerusalem had no way of knowing the exact day of Passover.
To be sure of keeping the festival properly, they observed both possible dates.
The custom became general in time to double up the observance of festival days outside the land of Israel.
Today, when the Jews have possessed an exact calendar for nearly a score of centuries, when you can telephone Jerusalem from New York or from Tokyo, when the only doubt left about the moon is whether the USA or the USSR will land on it first.
Jews outside Israel still double up the festival days. In such things we tend to be a bit conservative.