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. There are different types of calendars, like the Gregorian calendar, which is the most widely used today, and the lunar calendar, which is based on the phases of the moon. Calendars can be found in various forms, including paper planners, wall calendars, and digital apps. They often include important dates and holidays, making it easier for people to remember special occasions. Using a calendar can help improve time management and productivity in daily life.