[An extra month is added,] making the year full, because of three factors: a) the vernal [spring] equinox; b) the ripening [of the barley crop], and c) the blooming of the fruit trees.What is implied? When the court calculates and determines that the vernal equinox will fall on the sixteenth of Nisan or later, the year is made full. The month that would have been Nisan is made the second Adar, and thus Pesach will fall in the spring. This factor [alone] is sufficient for the court to make the year full; other factors need not be considered.

The Fast of Esther was established on the basis of a statement in the Talmud (Megillah 2a) that the 13th of Adar is “a time of gathering for everyone.” The commentaries explain (Rosh, ad loc., sec. 1; Ran, 1b) that everyone gathers for the Fast of Esther and village dwellers come to the city to recite prayers of repentance and supplication. This is because on that day the Jews gathered to defend themselves and were in need of mercy. Therefore this day was established for the reciting of prayers of repentance and supplication. Village dwellers are obliged to come to the city on the Fast of Esther so that they may join with their fellow Jews in the reciting of the penitential prayers, for this is the meaning of the Sages’ statement that the 13th is “a time of gathering for everyone.” The Holy One Blessed is He loves when people gather in the cities to recite penitential prayers with the congregation. This is because “in a multitude of people is the glory of the king” (Mishlei 14:28). Through these prayers they arouse abundant mercy on the part of the Heavenly host.