A "calendar year" refers to 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 calendar system in the world. Each calendar year consists of 365 days, except for leap years, which have 366 days. Leap years occur every four years to account for the extra time it takes for the Earth to orbit the sun. This system helps maintain alignment with the seasons and is essential for activities like agriculture and holiday planning.