A calendar is a system used to organize days, weeks, and months of the year. It helps people keep track of time and plan events, such as birthdays, holidays, and appointments. Most calendars show the days of the week, which usually start with Sunday or Monday, and they are divided into months like January, February, and so on. There are different types of calendars, including 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 formats, such as wall calendars, digital calendars on phones, or even planners that people carry around.