A calendar year is a period of 365 days, or 366 days in a leap year, that is used to organize time. It typically starts on January 1 and ends on December 31. The calendar year is divided into twelve months, each with a varying number of days, and is based on the Earth's orbit around the Sun. The concept of a calendar year helps people plan events, track seasons, and manage schedules. Different cultures may have their own calendars, such as the lunar calendar or the fiscal year, but the Gregorian calendar is the most widely used worldwide.