The Gregorian calendar is composed of 12 months, named in order, January, February, March, April, May, June, July, August, September, October, November, and December. The 12 months form a year. A year is either 365 or 366 days in length, where the extra day is used on ``leap years'', defined as years evenly divisible by 4, except for centesimal years divisible by 400. Each month has a fixed number of days, except for February, the length of which varies by a day depending on whether or not the particular year is a leap year.