Charles Choi has a nice post on simulating the Unix at command with Emacs. Back in the old days, Choi used to use the Unix at command to notify himself of various events. In macOS, the at command is disabled by default because its use can deplete the battery. Old habits die hard though, and Choi wanted a way to get the same behavior.
He started with the Emacs run-at-time function that does roughly the same thing as the at command except it runs an Emacs function instead of a command line function. Choi filled that gap with a macOS shortcut that he calls nota. The idea is that a small Emacs function schedules nota with the desired message at whatever time you want.
If you’d like to use his workflow, his post has a link to his shortcut that you can download. Of course, the shortcut bit works only with macOS but I doubt it would be hard to adapt to any Unix-like system. As far as I can see, the shortcut mechanism is merely a convenience and could probably be replaced with something like the Unix message command.
Regardless, if you’re looking for a handy way to serve a notification at a certain time and, like Choi and me, you have Emacs running all the time, take a look at his post.