Original: Because of the roughly eleven-day difference between twelve lunar months and one solar year , the length of the Hebrew calendar year varies in a repeating 19-year Metonic cycle of 235 lunar months , with an intercalary lunar month added according to defined rules every two or three years , for a total of 7 times per 19 years .
Simplified: Because of the roughly 11 day difference between twelve lunar months and one solar year , the calendar repeats in a 19-year cycle of 235 lunar months , with an extra lunar month added once every two or three years , for a total of 7 times per 19 years .