"This month shall be to you the beginning of months"—Exodus 12:2.
We are commanded to establish a calendar and calculate its months and years. The months are lunar months, with a new month established when the new moon appears; the years follow the solar seasonal cycle, necessitating the periodic addition of an extra (thirteenth) month to a year – which then becomes a "leap year" – because twelve lunar months are several days short of a solar year. This mitzvah is known as Sanctifying the New Moon.
This mitzvah is entrusted to the Jewish Supreme Court that presides in Israel. Unlike the counting of six days and then observing the Shabbat, a mitzvah that is incumbent upon every individual, no individual can unilaterally decide that a new month has arrived simply because he espied the new moon, and no individual can decide to add a month to the calendar based on his personal (even Torah-based) calculations.
Only the Supreme Court can make these calculations, and only in the Land of Israel. We follow the rulings issued by the Supreme Court in Israel even if they inadvertently established the "wrong" day as the New Moon, even if they did so under duress.
In the event that there are no qualified rabbis remaining in Israel, these calculations can be made, and months and leap years established, by a court that was ordained in Israel—even if it finds itself in the Diaspora.
Today we no longer sanctify the months based on the testimony of witnesses who saw the new moon, because there is no longer a sitting rabbinical Supreme Court in Israel—much as we no longer offer sacrifices, because we lack a Holy Temple.
But under no circumstances can an individual or court outside of Israel establish a new month or a leap year. Our calculations today in the Diaspora are only to determine which days the Court in Israel established as the New Moon, and which years they established as leap years.
[Editor's Note: Nachmanides asks, if so, how do we have holidays and a calendar today, when there is no rabbinical Supreme Court in Israel? He answers that there is a tradition that Hillel the Prince, who resided in Israel, established a calendar until the arrival of Moshiach, and sanctified all the new months and leap years until that time. Therefore, we can use our calculations to determine exactly what he previously established.]
Some laws associated with this mitzvah:
- The extra month added to a leap year is the one contiguous to the month of Passover—i.e. Adar.
- The establishment of new months and leap years must be done during daylight hours.
- A year must be comprised of complete months; a month must be comprised of complete days.
