The lo ADU Rosh calendar rule names which three weekdays for Rosh Hashanah?

The lo ADU Rosh rule says that Rosh Hashanah cannot fall on Sunday, Wednesday, or Friday. The mnemonic is written לא אד״ו ראש, in which the Hebrew letters alef, dalet, and vav represent the first, fourth, and sixth days of the week.

In the traditional Hebrew weekday count, Sunday is day one, Wednesday is day four, and Friday is day six. The phrase therefore identifies prohibited weekdays rather than listing the days on which the holiday is permitted to begin.

The rule is one of the postponement principles used by the fixed Hebrew calendar. It helps prevent difficult interactions between Rosh Hashanah, Yom Kippur, and other calendar observances. For example, the calendar’s weekday restrictions are designed to avoid certain back-to-back arrangements involving Shabbat and Yom Kippur.

A common mix-up is to answer with the allowed weekdays—Monday, Tuesday, Thursday, or Saturday. Those are the remaining possibilities. The stored answer gives the three weekdays named by the lo ADU Rosh mnemonic: the days specifically ruled out for Rosh Hashanah.