Emacs provides the functions of a desk calendar, with a diary of planned or past events. It also has facilities for managing your appointments, and keeping track of how much time you spend working on certain projects.
To enter the calendar, type M-x calendar; this displays a three-month calendar centered on the current month, with point on the current date. With a numeric argument, as in C-u M-x calendar, it prompts you for the month and year to be the center of the three-month calendar. The calendar uses its own buffer, whose major mode is Calendar mode.
Mouse-3 in the calendar brings up a menu of operations on a particular date; Mouse-2 brings up a menu of commonly used calendar features that are independent of any particular date. To exit the calendar, type q.