📅 A calendar is a system of organizing and measuring time, typically divided into days, weeks, months, and years. It helps individuals keep track of dates, appointments, events, and important milestones. Calendars often display the days of the week and months in a grid format, allowing people to easily plan and schedule activities. They can be physical paper-based versions or digital formats accessible through various devices and applications. Calendars are widely used for personal, professional, and societal purposes to manage time effectively and stay organized.