If you have a diary entry for an appointment, and that diary entry begins with a recognizable time of day, Emacs can warn you, several minutes beforehand, that that appointment is pending. Emacs alerts you to the appointment by displaying a message in the mode line.
To enable appointment notification, you must enable the time display feature of Emacs, M-x display-time (Section 5.3). You must also add the function appt-make-list to the diary-hook, like this:
(add-hook 'diary-hook 'appt-make-list)
Adding this text to your .emacs file does the whole job:
(display-time) (add-hook 'diary-hook 'appt-make-list) (diary 0)
With these preparations done, when you display the diary (either with the d command in the calendar window or with the M-x diary command), it sets up an appointment list of all the diary entries found with recognizable times of day, and reminds you just before each of them.
For example, suppose the diary file contains these lines:
Monday 9:30am Coffee break 12:00pm Lunch
Then on Mondays, after you have displayed the diary, you will be reminded at 9:20am about your coffee break and at 11:50am about lunch.
You can write times in am/pm style (with 12:00am standing for midnight and 12:00pm standing for noon), or 24-hour European/military style. You need not be consistent; your diary file can have a mixture of the two styles.
Emacs updates the appointments list automatically just after midnight. This also displays the next day's diary entries in the diary buffer, unless you set appt-display-diary to nil.
You can also use the appointment notification facility like an alarm clock. The command M-x appt-add adds entries to the appointment list without affecting your diary file. You delete entries from the appointment list with M-x appt-delete.
You can turn off the appointment notification feature at any time by setting appt-issue-message to nil.