How would I go about warning users before a scheduled reboot happens?

I’ve been trying to figure out the best way to warn my users of the upcoming reboot, any help is appreciated.

There are mods out there which send any user a message upon log in so just get one of those and give a welcome message saying scheduled maintenance due at xx:xxpm EST or w/e

How are you currently rebooting?
If you have a script scheduled using cron you could alter the script to have the server say something (I forget the syntax, something like mineos.console ‘server name’ ‘say Reboot is in 15 mins’; someone will have to clarify) then after that line you sleep for 900secs then do the stop command then start command or full server reboot.