A calendar is a chart that organizes the year into days, weeks or months. It's also a list of appointments or activities, like the calendar for the club that lists when the bands are playing.
A calendar might hang on your refrigerator, where you can scribble your appointments on the squares. If you have something scheduled every day of the week, your calendar is full. In that case you might keep track of your dates with a calendar on your computer (or phone). Don't misspell it with an er ending, or you're referring to a machine that presses cloth. If you remember that "DAys" are in calenDArs, then you'll know to end it with a DAr.