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. A calendar is also a physical record (often paper) of such a system. A calendar can also mean a list of planned events, such as a court calendar or a partly or fully chronological list of documents, such as a calendar of wills.

The common theme of making calendars is the desire to organize time units to meet the needs and businesses of society. In addition to practical purposes, the organizational process offers a sense, how illusory, also the understanding of and time itself. Calendars thus serve as a link between humanity and the cosmos. It is no wonder that calendars held a sacred status and served as a source of social order and cultural identity. Calendars provided the basis for planning agricultural, hunting and migration cycles, for divination and prediction, and for maintaining cycles of religious and civil events. Regardless of their scientific refinement, calendars should ultimately be judged as social contracts, not as scientific treatises.