1,237. Beginning Hilchos Kiddush HaChodesh

Hilchos Kiddush HaChodesh 1:1

The months of the Jewish year are lunar months, as is derived from Numbers 28:14, “the burnt offering of the month when it is renewed,” and Exodus 12:2, “This month will be the first of months for you.” The Sages said that God prophetically showed Moshe a vision of the moon and said, “When you see the moon looking like this, sanctify it.” The years we follow, however, are solar years, as we see from Deuteronomy 16:1, “Keep the month of spring” (i.e., Passover always falls in the spring). [Editor’s note: Judaism, therefore, observes what is called a “lunisolar” calendar. This is unlike the Muslim calendar, which is strictly lunar, which is why the month of Ramadan can fall at any time of year.]

Hilchos Kiddush HaChodesh 1:2

A solar year is approximately 11 days longer than a lunar year. In order to correct the alignment between the lunar and the solar calendars, an additional month is added whenever the discrepancy between them reaches approximately 30 days. This causes a year to have 13 months; such a year is called a “full” year. This is done because it is impossible to have a 12-month year with the appropriate number of days. Numbers 28:14 says “of the months of the year,” regarding which the Sages stated that we are to count the months of the year, not the days of the year.