Months have either 4 or 5 normal weeks, i.e. 28 or 35 days. Patterns are usually quarter-based, i.e. 35:28:28 = 5:4:4, 28:35:28 = 4:5:4 and 28:28:35 = 4:4:5, but can be totally irregular (similar to ISO weeks in the Gregorian calendar).
Four times 91 makes 364 days, thus a leap week has to be introduced which may make one month irregularly 5 weeks = 35 days or 6 weeks = 42 days long in leap years.
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