One of the great controversies in Jewish history was the setting of the calendar. Though the Bible declared that certain holy days were associated with certain times of the year and particular months, it often referenced Babylonian months (Aviv) or times (count seven weeks from the first day of Passover to reach Shavuot). How would the months be determined?

The first attempt at setting standardized times for observance came with the rise of the rabbis around the beginning of the common era. The high court—or Sanhedrin—would declare the new moon. Signal fires would be set on hilltops (and messengers would be sent) to announce the beginning of the month. As Jews spread throughout the world, the limitations of this system quickly became apparent. So, in about 350 C.E., the rabbis learned from the Greeks and Romans and used the science and math of their day to calculate the calendar.

However, a purely lunar calendar would result in holidays moving off of their appointed times and falling in the wrong seasons. Corrections had to be made. Jews in Babylonia and Jews in Israel (around 921 C.E.) disagreed over the system of calculations, over the length of the months, and over the way of achieving the corrections necessary to keep the months in sync with the seasons. A certain Aaron Ben Meir in Israel insisted on using his conventions for determining the months, but the rabbis in Babylon ultimately won out and excommunicated Ben Meir! Ultimately, the matter was resolved (not without some later controversy), giving rise to our present system of adding an entire leap month periodically. Though the Jewish calendar appears to jump around the Gregorian system, both reflect alternative ways of ensuring that months and holidays occur with the seasons.

Judaism’s determination to discern an order to the world—and to order human lives accordingly—serves to create a unique Jewish identity. Our holidays blend history with nature, the seasons with celebration, and the cycles of our lives with holy days that provide opportunities for personal growth.

Source: The Great jewish Calendar Controversy

— Rabbi Ron Stern