1,282. What Makes a Month Malei or Chaseir

Hilchos Kiddush HaChodesh 8:1

A lunar month is 29 and a half days plus 793 chalakim, as was previously explained. It is not possible for Rosh Chodesh to start in the middle of the day, as this would cause part of the day to be in the previous month and part of the day to be in the new month. This is not permitted based on Numbers 11:20, which refers to “a month of days.” Our oral tradition understands this to mean that we are to count the days of a month and not the hours of a month.

Hilchos Kiddush HaChodesh 8:2

Because of this, some lunar months are made “chaseir” (“deficient,” i.e., short a day) and others are made “malei” (“full”). A month that is chaseir has only 29 days even though a lunar month is actually a few hours longer than this. A month that is malei has 30 days even though a lunar month is actually a few hours shorter than this. By doing this, the months end up being calculated based on whole days and not based on hours.