Answer:
June is the answer
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January - 31 days
February - 28 days in a common year, 29 days in leap years
March - 31 days
April - 30 days
May - 31 days
June - 30 days
July - 31 days
August - 31 days
September - 30 days
October - 31 days
November - 30 days
December - 31 days