A calendar is a systematic and structured documentation of the units of time for the purpose of estimating or calculating time over an extended period. Simply put, it is a record of the various units of time, namely days, months and years. A day is the smallest unit in the calendar. Calendars have existed in human civilizations for centuries. Historically, calculations of days, months and years in a calendar were determined by astronomical observations and events. The sun and the moon cycles were considered vital for creation and comprehension of early calendars. The movement of the earth from one fixed position to another was considered important for the depiction of a year, which is approximately 365.242190 days, although it may vary slightly. Similarly, the movement of the moon around earth (from one new moon to the next) was used to determine the synodic month, which is approximately 30 days, although it may vary. Some calendars are based on abstract continuously occurring cycles alone.

There are now about 40 calendars used throughout the world. The Christian, or Gregorian, calendar is now the most widely used calendar. Some of the concepts of month in this calendar have been derived from the Julian calendar. The Gregorian calendar had been introduced by Alvysius Lilius, a physician from Naples, and later adopted by Pope Gregory. The tropical year is 365 47/400 days, with 97 leap years in every 400 years. Some calendars, like the Islamic calendar, are based completely on lunar cycles, while others like the Jewish calendar are based both on solar and lunar cycles.

Calendars have been devised to serve the social, religious and occupational requirements of people of various cultures and societies for millenia. Even the aggressive and brutal Vikings of Scandinavia recognized the significance of calendars to plan their daily activities and living. In antiquity calendars enabled people to plan for agriculture, hunting and irrigation cycles, and record the dates of significant religious and social events. A calendar has acquired greater significance today. Scheduling one's daily life has become necessary for survival.