The Emacs diary keeps track of appointments or other events on a daily basis, in conjunction with the calendar. To use the diary feature, you must first create a diary file containing a list of events and their dates. Then Emacs can automatically pick out and display the events for today, for the immediate future, or for any specified date.
Although you probably will start by creating a diary manually, Emacs provides a number of commands to let you view, add, and change diary entries.