Sifrei Mitzvot untuk Imamat 23:25

These six days on which Scripture prohibited labor—being the first and seventh days of Passover, the first and eighth days of Sukkot, the day of Shavuot and the first day of the seventh month (Rosh Hashanah)—are called holidays (yamim tovim). The [degree of] cessation of work is equal in all of them, in that all work is prohibited except for labor that is for the sake of eating—as it is stated (Exodus 12:16), "only anything that is eaten by any person."

To offer up an additional sacrifice on the New Year, as it is said, “And in the seventh month, on the first day of the month…. ye shall offer up….” (Num. 29:1-6).

To not do work on the first day of Tishrei: To not do work on the first day of the month of Tishrei, as it is stated (Leviticus 23:24-25), “In the seventh month on the first of the month, etc. All work of labor shall you not do.” And the seventh month is called “Tishrei,” as it is the seventh from Nissan, which is the new year for [the count of] months. They, may their memory be blessed, said (Rosh Hashanah 2a) that this day, the first of Tishrei, is called the new year to count from it the sabbatical years and the Jubilees, and we also count plantings and vegetables from it. In the Gemara in Tractate Rosh Hashanah 8, 12a, the Sages elucidated what are the laws [that pertain] to their saying that it is considered the new year for these things. They also said there (Rosh Hashanah 16a) that on this day all the inhabitants of the world are judged for their deeds. They explained His providence of the deeds of each and every [individual] and on the species more generally, such that metaphorically all people pass before Him like bnei maron (in an orderly procession) — meaning, one at a time and not mixed together.