A calendar is a system used to organize days, months, and years. It helps people keep track of time and plan events, such as birthdays, holidays, and appointments. Most calendars are divided into months, which are further divided into weeks and days. The most common type is the Gregorian calendar, which is used by many countries around the world. Calendars can be found in various forms, including paper versions, digital apps, and wall charts. They often include important dates, such as New Year's Day and Christmas, making it easier for people to remember and celebrate special occasions.