A system that gives a name to each day. These day names are generally distinct, but not always. For example, the day of the week is a calendar, in a trivial sense, with infinitely many days having the same day name. The design of calendars has been driven by human needs varying over millennia, so the study of calendars is intricately bound to history, astronomy, and religion. Ancient calendars were based on simple observations of phenomena such as the waxing and waning of the Moon, the change in seasons, or the movement of heavenly bodies. Because observational methods suffer from the vagaries of weather and chance, modern calendars tend to be based solely on arithmetical rules, distanced from their motivation in nature.