Let's say Tuesday December 20, I need to leave the house at 10 AM. I'd like a pop-up notifier that reminds me (and stays on the screen until I've hit "ok"). What program works best for this?
There are several methods to accomplish this from command line tools to calendars. What features are you looking for ?
Evolution has a notification system.
In Evolution Options -> Reminders
If that is not going to work out for you, please describe why not or what features you need/desire.