Calendar terms refer to the various units of time used to organize and measure days, weeks, months, and years. Common terms include day, which is a 24-hour period; week, consisting of seven days; and month, typically lasting 28 to 31 days. A year is the time it takes for the Earth to complete one orbit around the Sun, usually 365 days, or 366 in a leap year. Different calendars may use unique terms and structures. For example, the Gregorian calendar is the most widely used today, while the lunar calendar is based on the phases of the Moon. Understanding these terms helps in planning events and managing schedules effectively.