Throughout history, people have used various systems to organize the year into units such as days, weeks, and months. A calendar’s primary function is regulating and organizing human activities; the word derives from the Latin calendarium or calendra, “account book,” and kalendae or “calends,” the new moon and first day of the Roman month.
Unlike the day, the month, and the year, the week is not based on any astronomical event.
The modern calendar has 12 months of 30 or 31 days each (February has 28, sometimes 29). The calendar year has 365 days, which is about how long it takes the Earth to circle the Sun once. That makes it a solar calendar.
The oldest calendars were used to figure out when to plant, harvest, and store crops. These were often “lunar calendars,” based on the number of days it took the Moon to appear full and then dwindle away again.
Many calendars have religious origins.