A calendar is a tool used for writing history and for planning things that have not happened yet using days, called calendar dates. The dates are usually made based on how things in the sky seem to move, like the Sun or moon. By knowing what day something happened or will happen, people have an easier life.
The calendar most used today is the Gregorian Calendar.
A calendar is also something that shows the system in writing. These can be on paper or software.
The term is also used to mean a set of planned events (for example, court calendar).