It is taught in a baraita: Rabbi Shimon ben Elazar said: Ezra enacted for the Jewish people that they should read the portion of the curses that are recorded in Leviticus before Shavuot and the portion of the curses that are recorded in Deuteronomy before Rosh HaShana. The Gemara asks: What is the reason for this? Abaye said, and some say that it was Reish Lakish who said: In order that the year may conclude together with its curses, and the new year may begin without the ominous reading of the curses.
The Gemara asks: Granted, with regard to the curses that are recorded in Deuteronomy, there is relevance to the reason: In order that the year may conclude together with its curses, for Rosh HaShana is clearly the beginning of a new year. However, with regard to the curses that are recorded in Leviticus, what relevance does that reason have? Is that to say Shavuot is a new year? The Gemara answers: Yes, indeed, Shavuot is also a new year, as we learned in a mishna (Rosh HaShana 16a): And on Shavuot, divine judgment is made concerning the fruit of the trees, which indicates that Shavuot also has the status of a new year.
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There are four new years: The first of Nisan is the new year for kings and for festivals. The first of Elul is the new year for the tithe of beasts. Rabbi Elazar and Rabbi Shimon say: the first of Tishri. The first of Tishri is the new year for years, for shmitta and jubilee years, for planting and for [tithe of] vegetables. The first of Shevat is the new year for trees, according to the words of Bet Shammai. Bet Hillel says: on the fifteenth of that month.
At four times of the year the world is judged: On Passover judgment is passed concerning grain; on Shavuot concerning fruits that grow on a tree; on Rosh HaShana, all creatures pass before Him like sheep [benei maron], as it is stated: “He Who fashions their hearts alike, Who considers all their deeds” (Psalms 33:15); and on the festival of Sukkotthey are judged concerning water, i.e., the rainfall of the coming year.