Halakha Keluaran 9:31: Mishneh Torah, Sanctification & The Sabbath Epistle

For three indicators do we make the year a leap year. For the season, for the Spring [state of the barley] and for the fruit of the trees. How is that? The court calculates and determines – if the season of Nissan (when the sun enters Aries) will be on the sixteenth of Nissan or after that time, they make that year into a leap year. And they make that Nissan [into] Second Adar, so that Passover will be during the time of Spring. And we may rely on this indicator and make leap years, and [then] not be concerned about another indicator.

When we investigate the Torah’s year, we find written “This month shall be for you the beginning of months” (Exodus 12:2), so it is first of the months of the year. It is also written “This day you depart, in the month of ripening” (ibid. 13:4), and “Observe the month of ripening” (Deuteronomy 16:1). The explanation is that Israel counts by lunar months, and the month in which the barley ripens in the Land of Israel is the first of the year’s months. The beginning of that month is the beginning of the year, whether the equinox has passed or not. However, in order to perform the waving of the Omer ⁷⁸ The Omer waving took place on the second day of Passover, the sixteenth of Nisan (Leviticus 23:10–11). the court should ensure that Passover will occur when the barley has ripened.⁷⁹ There are exceptions to this requirement. See Ibn Ezra’s commentary to Exodus 12:2 and to Deuteronomy 15:1. Most years the ripening coincides with the equinox, but sometimes they are separated slightly because of an abundance of rain or because of drought.