A calendar year is a period of 12 months that begins on January 1 and ends on December 31. It is used to organize time for various purposes, including business, education, and personal planning. The calendar year is based on the Gregorian calendar, which is the most widely used civil calendar today. Each calendar year typically consists of 365 days, except in a leap year, which has 366 days to account for the extra time it takes for the Earth to orbit the Sun. Leap years occur every four years, with the exception of years divisible by 100 but not by 400.