Simplify the following text for a Grade 9 native English-speaking person.
Original: The Jewish calendar is a lunisolar calendar , or fixed lunar year , based on twelve lunar months of twenty-nine or thirty days , with an intercalary lunar month added seven times every nineteen years ( once every two to three years ) to synchronize the twelve lunar cycles with the slightly longer solar year .
Simplified: The `` modern '' form is a fixed arithmetic lunisolar calendar . 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 .