1,342. Alternating Full and Deficient Months

Hilchos Kiddush HaChodesh 18:8

We have a tradition, transmitted by our Sages all the way back to Moshe, that if the moon is not seen at the start of the month for several months, the court follows a pattern alternating a full month of 30 days and a deficient month of 29 days. They would calculate and establish alternating full and deficient months but they would not sanctify them because sanctification requires that the moon be witnessed. Sometimes the court would have one full month follow another, or one deficient month follow another, all based on their astronomical calculations.

Hilchos Kiddush HaChodesh 18:9

The idea of the calculations is that it is possible for the moon of the coming month to be seen at its proper time (on the night of the 29th day), or on the night after the added (30th) day, but not beforehand, on the night of the 28th day. Through the aforementioned calculations, it is possible for one to determine when the moon will and will not be visible. Such calculations are employed to determine if we need two consecutive full months or two consecutive deficient months. There should never be fewer than four full months in a year, nor more than eight. When a full month was established based on calculations, a festive meal was held, as was discussed in halacha 3:7.