A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months, and years. A date is the designation of a single, specific day within such a system.

Days of the Week

The names of the days of the seven-day week in many languages are derived from the names of the classical planets in Hellenistic astrology, which were in turn named after contemporary deities, a system introduced in the Roman Empire during the Late Antiquity. In some other languages, the days are named for corresponding deities of the regional culture, beginning with Sunday or Monday.

Months of the Year

A month is a unit of time used with calendars, which is approximately as long as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months (lunations) are synodic months and last approximately 29.53 days. Researchers have deduced that people counted days in relation to the Moon's phases from excavated tally sticks as early as the Paleolithic age.