- Shabbat and Holidays
- Rosh Chodesh
A new month does not automatically begin when the moon reappears. Rather, the beit din sanctifies the month, as it says, "This month shall mark for you" (Shemot 12:2). God showed Moshe the configuration of the moon in its renewed state and said, "This testimony shall be given over to you" (rh 22a). That is, witnesses must come before you and testify that they saw the new moon, and you shall sanctify the month based on their testimony.
After Moshe’s death, the authority to establish the Jewish calendar was conferred upon the Beit Din Ha-gadol of every generation (the high Jewish court that functioned in earlier times; the Sanhedrin), on condition that its judges received semikha (rabbinic ordination) from another who has received semikha in an unbroken chain that originates with Moshe himself. Such ordination can only be transmitted in the Land of Israel (mt, Laws of Courts 4). If a time comes when the Jews are unable to set the months by way of a beit din, the halakha states that they must do so using mathematical calculations.
Thus, even though the lunar cycle is a natural phenomenon, the renewal of the moon does not, by itself, sanctify the month. Rather, the Jewish people sanctify the months, and by doing so enable the holiness of time to reveal itself. This explains why the Sages decided to end the middle berakha of the Musaf prayer on Rosh Ĥodesh with the words "Blessed are You, Lord, Who sanctifies Israel and the New Moons" (Ber. 49a). Perhaps this is also why the first mitzva the Torah commanded the Jewish people was the mitzva of sanctifying the new month (Shemot 12:2), as this mitzva showcases Israel’s unique sanctity – that the holiness of time reveals itself through them.
 If witnesses who saw the new moon on the night of the thirtieth day of the month come before the beit din the next day, the court sanctifies the month on that day, transforming the thirtieth day of the previous month into the first day of the new month: Rosh Ĥodesh. They then immediately offer the special Rosh Ĥodesh sacrifices. Consequently, the previous month is rendered an "incomplete month," as it had only 29 days. If, however, no witnesses arrive on the thirtieth, Rosh Ĥodesh is the next day – the 31st – and the previous month is rendered a "full month," consisting of a full thirty days. In such a case, the beit din does not need to hear witnesses’ testimony or declare the beginning of the new month, as in any case there are only two days on which Rosh Ĥodesh can possibly occur; if no witnesses come on the first day, Rosh Ĥodesh automatically falls out on the next day (mt, Laws of Sanctifying the Month 2:8).