Months of 29 or 30 days begin on days of astronomical New Moons, with an intercalary month being added every two or three years.
(Months of 29 or 30 days; begin; T:on days of astronomical New Moons) (an intercalary month; being added; T:every two or three years)
Months of 29 or 30 days an intercalary month
begin being added
T:on days of astronomical New Moons
T:every two or three years