A calendar is a tool for organizing days. People use calendars to say when something happened, and for planning things that have not happened yet. The calendar gives the days names and numbers, called calendar dates. The "dates" are usually made based on how things in the sky seem to move. The year and month are based on motions of the Sun and moon. By knowing what day something happened or will happen, people have an easier life. Since the beginning of history, knowing when the seasons would start for crops was very important for farmers and people who eat.