1,285. Calculating Whether the Months of a Year are Malei or Chaseir

Hilchos Kiddush HaChodesh 8:7

The way to determine whether the months of a year will be chaseir, malei, or alternate is as follows: First, determine the day of Rosh Hashana in the desired year, as was explained in chapter 7. Then determine the day Rosh Hashana will be in the year after that. Next, count the number of days in between them not including the days of Rosh Hashana in these two years. If there are two days between them, the months be chaseir; if three days, the months will alternate in order; if four days, then the months will be malei.

Hilchos Kiddush HaChodesh 8:8

This only applies when the desired year is a regular (non-leap) year. For a leap year, four days means that the months will be chaseir, five days means that the months will alternate in order, and six days means that the months will be malei.