Beitzah 16A

כל מזונותיו של אדם קצובים לו מראש השנה ועד יום הכפורים, חוץ מהוצאת שבתות והוצאת יום טוב והוצאת בניו לתלמוד תורה, שאם פחת ⁠— פוחתין לו, ואם הוסיף ⁠— מוסיפין לו.

A person’s entire livelihood is allocated to him during the period from Rosh HaShana to Yom Kippur. During that time, as each individual is judged, it is decreed exactly how much money he will earn for all his expenditures of the coming year, except for expenditures for Shabbatot, and expenditures for Festivals, and expenditures for the school fees of his sons’ Torah study. In these areas, no exact amount is determined at the beginning of the year; rather, if he reduced the amount he spends for these purposes, his income is reduced and he earns that much less money in that year, and if he increased his expenditures in these areas, his income is increased to ensure that he can cover the expense. Therefore, one may borrow for these purposes, since he is guaranteed to have enough income to cover whatever he spends for them.

אמר רבי אבהו, מאי קראה: ״תקעו בחדש שופר (בכסא) ליום חגנו״, איזהו חג שהחדש מתכסה בו ⁠— הוי אומר זה ראש השנה, וכתיב: ״כי חק לישראל הוא משפט לאלהי יעקב״.

R' Abbahu said: What is the verse from which this dictum is derived? The source is: “Blow the shofar at Rosh Chodesh, at the concealed time for our Festival day” (Psalms 81:4). On which Festival is Rosh Chodesh concealed? You must say that it is Rosh HaShana, which occurs on the 1st of the month, when the moon is not yet visible, while the moon is visible during the other Festivals, which occur in the middle of the month. And it is written in the next verse: “For it is a statute [ḥok] for Israel, a judgment of the God of Jacob” (Psalms 81:5).