The number of weeks in a month is not constant throughout the year. Most months have 4 weeks, but some have 5.
Here's a breakdown:
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Months with 4 weeks: April, June, August, September, November, and January, March, May, July, October, December (all have 30 or 31 days).
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Months with 5 weeks: February (usually 28 days) can sometimes have 29 days in a leap year.
Since February is the only month with a variable length, most months have 4 weeks.