1,286. Checking One's Calculations

Hilchos Kiddush HaChodesh 8:9

Let’s say that one wants to know the order of the months (malei and chaseir) of the current year. If Rosh Hashana fell on Thursday and it is a regular (non-leap) year, and the next Rosh Hashana will fall on Monday, that means there are three days between them and it is a year whose months are in order (i.e., alternating between malei and chaseir). If the Rosh Hashana of the next year falls on Tuesday, the months of the year will be malei. If Rosh Hashana falls on Shabbos in the current year and on Tuesday in the subsequent year, the months of the year will be chaseir. The same ideas are employed when calculating the months of a leap year, as was described in halacha 8:8.

Hilchos Kiddush HaChodesh 8:10

There are signs that one can rely upon in order not to make mistakes when calculating the order of a year’s months. These are based on the basic rules of the fixed calendar and the designation of which days can be set as Rosh Hashana, as opposed to the days on which it will be postponed. When Rosh Hashana is observed on a Tuesday, the year will be “in order” of malei and chaseir, whether it is a regular year or a leap year. When Rosh Hashana is observed on Shabbos or Monday, the year will not be “in order,” again regardless of whether it is a regular year or a leap year. If Rosh Hashana is on a Thursday, then if it is a regular year, the months cannot be chaseir and if it is a leap year, the months cannot be “in order.”