The paper considers the calendar as a link between the cosmos and mankind, and itintroduces it as an instrument in studying culture. It uses the concept of calendars/calendar systemsas a criterion for recognition and formation of culture in general. Starting from an assumption thatthe calendar is a structurally organized system of events or holidays, it analyses the basic units of acalendar: day, month and year, and distinguishes a calendar holiday from a non-calendar holiday.It states that the calendars are a structural list of collective memory within a social group, wherethis memory is described in cyclical categories – calendar holidays. Furthermore, considering thatthe initial epoch of year counting may be different in different cultures, it discusses how culturalself-awareness is expressed through the epoch of the calendar era. Finally, it explores how and towhat extent the formation, interaction, and reforms of calendars and their systems reflect the changein culture. The paper concludes that calendars and their systems should be used as a criterion indefining culture.