Origins of the Jewish calendar
Sages, witnesses, and new moons formed today’s lunar calendar

Rabbi Fried,
We all get Jewish calendars at the grocery store or in the mail and take them for granted. What are the origins of the Jewish calendar? I’ve been wondering about this for years, and hope you can provide some insight.
Marvin G.

Dear Marvin,
The very first mitzvah that the Jewish people were commanded while still in Egypt was to sanctify the new month. “This renewal [of the moon] shall be for you the beginning of the months, it shall be for you the first of the months of the year.” (Exodus/Shemos 12:1-2)
This means that the Jewish people are not to simply calculate our dates. We need to sanctify the first day of each month, which is called Rosh Chodesh, or the “head of the month.” From the time of Moses, and for nearly a thousand years, the high Jewish court, or Sanhedrin, calculated and sanctified the new month. Each month, the Sanhedrin in Jerusalem would wait for two witnesses to appear, stating they had observed a new moon. After testing the veracity of the witnesses, the court would establish the sanctity of Rosh Chodesh by the power vested in them, and by proclaiming “Mekudash, mekudash,” meaning, “It is sanctified, it is sanctified.”
From the time Joshua led the Jews into their land, conditions existed to carry out the sanctification of the new months. As long as the months were sanctified in this divinely ordained manner, there could not be an annual set “calendar.” Because each new month required witness testimony, no one could guarantee whether the current month would be 29 or 30 days. Jewish holidays, such as Passover, depended upon when the Sanhedrin proclaimed the new month.
Jews would wait to be informed when Pesach would “fall out” that year, an uneasy feat in a pre-electronic communicative world. A system of bonfires, lit atop mountains across Israel, would announce from Jerusalem when the new month was established. When saboteurs maliciously lit fires on the wrong days to mislead the people, the Sanhedrin had to send actual messengers by horseback across the country to inform all of the new month.
This process continued until one of the last generations whose leaders still had actual semicha, or ordination, through an unbroken chain from Moses. That type of semicha was a prerequisite to sanctify the new month. Because this semicha was in danger of cessation, the entire institution of Rosh Chodesh and Jewish months were in danger. To ensure continuity of Jewish months, Hillel the Last and his court, who still held that form of semicha, calculated and sanctified all the coming months until the time of the messiah. By doing so, they established the first actual calendar, in the year 359 CE. From that time, and onward, they no longer needed to wait for witnesses. Rather, they relied upon calculations sanctified with the semicha power vested in them.
The Torah established that our months be lunar, or moon-based, as opposed to the solar, or Gregorian, calendar used today in most of the world. The Torah also commands that Passover always fall in the spring. (Deuteronomy/Devarim 16:1) These two commands, however, conflict with each other. For this reason, an extra month or leap year, was established to synchronize the solar with the lunar calendar, seven out of every 19 years. In this way, the months have remained successfully synchronized for more than 3,300 years since we received this commandment.
As an aside, it is fascinating to note the precision of our sages’ calculations. The Talmud (Rosh Hashanah 25a) and Maimonides (Code, Laws of Sanctifying the Moon 6:2-3) calculate the length of a solar month as 29.53059 days. A number of years ago, NASA made the following statement: “After years of research based on calculations using satellites, hairline telescopes, laser beams and super-computers, scientists at NASA have determined that the length of the ‘synodic month,’ i.e., the amount of time between one new moon and the next, is 29.530588 days.”
Jewish dates don’t arbitrarily fall out. They are calculated. We don’t just follow along with the flow of time. Rather, the Torah empowers us to actually change time and dates. The Torah puts us above time, and to live above time is to connect to the eternity of the Jewish people.

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