When you first learn about the appointment or event. Immediately put it in your calendar and set multiple reminders. Top priority. If you can’t do it then, add it anytime something reminds you of the event (if that ever happens).

It’s pretty much the only way I can remember appointments and events that aren’t too important. Of course, if it is important, that is all I’ll think about until the event, completely forgetting all other events until my reminder pops up.