The term 'month' comes from Middle English moneth; which comes from the Old English term 'monath'. Additionally it is akin to Old High German manod meaning month and Old English Mona, meaning moon. It in all probability is related to mê, an Indo-European term (meaning to measure), actually referring to the various moon's phases used as the measure of time.
A month may be thought of as that period into which any given year is divided. It is traditionally founded up on the actually phases of the moon. Presently, we typically use the Gregorian calendar wherein we have twelve months ranging from January to December. You could say a month implies a fixed interlude of 30 days, then 31 days, or some such alternation thereof. February has 28 or 29 days.
The Gregorian calendar is in effect a modification of the ancient Julian calendar.