A calendar is a chart or device which displays the date and the day of the week, and often the whole of a particular year divided up into months, weeks, and days.

A calendar is a particular system for dividing time into periods such as years, months, and weeks, often starting from a particular point in history.

You can use calendar to refer to a series or list of events and activities which take place on particular dates, and which are important for a particular organization, community, or person.