Apparently there are many types of calendars including hybrid 12 & 13 month long…

“In a Metonic calendar (a type of lunisolar calendar), there are twelve years of 12 lunar months and seven years of 13 lunar months. 

In the Babylonian and Hebrew lunisolar calendars, the years 3, 6, 8, 11, 14, 17, and 19 are the long (13-month) years of the Metonic cycle. This cycle forms the basis of the Greek and Hebrew calendars.“

https://en.m.wikipedia.org/wiki/Metonic_cycle