How do I create a shell script that would generate a system notification.

I am a student, and I have project to complete. The project is to create a power point presentation with the following guidelines:
You are the system admin for a medium sized Internet company that has its primary production web server running on UNIX/Linux. Your technical IT director would like for you to do a presentation on a system admin task that you have automated with the use of the shell (your choice of shell, Bourne, C, Korn, Bash, etc...). I would like to have a system notification that will come up if a user logs on within 4 hours of a schedualed system maintenece down time, and again 30 minutes before the down time, that would state " A schedualed system maintenence will begin at 2:00 am. Please please save and exit any web based applications by 1:55 am in order to avoid loss of data."
Can any of you give me an idea of how to create this type of shell script?

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