According to Longman Dictionary of Contemporary English, a calendar month is a full month or a specific date in one month that corresponds to the same day in the next. Months were first used in Mesopotamia and revolved around the cycles of the moon.
A calendar month can either be one whole month, such as October, or from a date in one month to the same day in the next. For example, July 23 to August 23 counts as a calendar month, even though days from two separate months are included.
The modern Gregorian calendar used today comprises 12 months, and each either has 28, 30 or 31 days. Every four years, the month of February has and extra day to make 29 because of the leap year.