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 Earth's orbit around the Sun, which takes approximately 365.25 days. Most calendar years are either common years, with 365 days, or leap years, with 366 days. A leap year occurs every four years to account for the extra 0.25 days in the Earth's orbit. This system helps maintain alignment with the seasons and is essential for the accurate tracking of time.