A calendar is a system for organizing days, months, and years. It helps people keep track of time and plan events. Most calendars are based on the cycles of the moon or the sun, with the Gregorian calendar being the most widely used today. It divides the year into 12 months, each with a specific number of days. Calendars can be physical, like wall or desk calendars, or digital, found on computers and smartphones. They often include important dates, such as holidays and birthdays, making it easier for individuals and organizations to schedule activities and manage time effectively.