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God created the luminaries – the sun and moon – and set them in motion. Ever since, people have used them to measure time. Days, nights, and years are established based on the sun. Months, the key to the timing of Jewish holidays, are based on the moon. This accords with the verse: “God said, ‘Let there be lights in the expanse of the sky to separate day from night; they shall serve as signs for the set times – the days and the years’” (Genesis 1:14).
Each lunar cycle, the moon completes one revolution around the earth. At the beginning of the month, we perceive the moon as just a sliver. Then it waxes, its apparent size gradually increasing until the middle of the month, when it appears as a complete circle, a full moon. During the second half of the month it wanes, disappearing from view entirely by the last day. Afterwards, it reappears as a thin sliver once more, indicating that a new month has begun. (We call the new moon the “molad,” which means birth.) A full lunar cycle takes about 29 and a half days. Therefore, some Jewish months are 29 days, and some are 30. In Talmudic times, when the Sanhedrin was still active, its members (personally ordained as rabbis in an unbroken chain going back to Moses) were the only ones authorized to sanctify the months. This means they announced the beginning of each month based on the testimony of witnesses who had seen the new moon.
In twelve lunar months, there are 354 days. However, the solar year is 365 days. Given that, were nothing done to keep the lunar and solar years in sync, eventually the holiday of Passover would no longer be celebrated in the spring! Since the Torah specifies that Passover must be in the springtime (Deuteronomy 16:1), the Sanhedrin was authorized to declare a leap year every two or three years. In a leap year, an extra month is added to the calendar (so instead of one month of Adar, we have Adar Aleph and Adar Bet). This ensures Passover will always take place in the spring.
The halakha states that if the Sanhedrin can no longer sanctify the months, the rabbis are authorized to do so using mathematical calculations. Towards the end of the Amora’ic period in 4119 (359 CE), when the majority of the Jews were in exile, Hillel II came to the conclusion that it was no longer viable for the Sanhedrin to sanctify the months. With the authority invested in him as the head of the Sanhedrin (having inherited the position in a direct line from Rabbi Judah the Prince), Hillel II, together with his Sanhedrin, calculated the months and years, sanctifying them for millennia to come. Ever since then, the Jewish people have been following the Jewish calendar as we know it today.
