Where to find or how to make a system for sending notifications to users?

Question

Notifications

Tell me, please, is there a program that can send messages on events from the database?
For example, in the database there is an entry Event - Date of the event - Time of the event. And it is necessary that specific users have a window with a reminder of this event displayed at the right time. And so that an icon hangs in the tray, clicking on which opens a window with a complete list of events for this user.
If there is nothing like this, advise where or where to look how to do it? (I just can't figure out how to do it better yet)
Thanks in advance to everyone.