Calendar, a system of dividing time into convenient periods of days, months, and years. The system used in most parts of the world is the Gregorian calendar.
The day and the year are based on solar (sun) time. One day equals the length of time it takes the earth to make one complete revolution on its own axis. One year is the approximate time it takes the earth to make one complete revolution around the sun. The month was originally the length of time it takes the moon to complete all its phases as it revolves around the earth. This lunar month of approximately 29 ½ days has been replaced in most calendar systems by a slightly longer month so that each month falls during the same season every year.
The week does not depend on movements of the earth or moon, or on anything else in nature. It is an arbitrary division of time developed for convenience.