A measurement of time defined as 30 days. According to the Gregorian calendar, a month can last anywhere between 28 and 31 days, with 12 months to make up a year. The months of April, June, September, and November hold 30 days, with January, March, May, July, August, October and December consisting of 31 days. February has 28 days, with 29 days every leap year (or every four years).