Display website maintenance notification few hours before the maintenance
===

Our website is on asp .Net. To bring down the website and display an offline message we use the app_offline.htm.

 But we want to notify our clients from a day before the time the website would be down, so that they can plan their activities accordingly. (I like the way stack over flow does it with a small notification on the home page).

I want to what is the best way to do it. Is it possible to get it done in a simple way like by some config file change or something?