While watching Superbowl 52, I finished my github project Maintenance Banner, aka mx_banner.php.
It lets an application administrator schedule maintenance event notifications for a web application in a database, and the web application can then automatically display those on the post-login page.
mx_banner.php is Apache-licenced and written in PHP with PDO and supports MySQL and Postgresql. There are only a few fairly generic SQL statements, so it should work also with SQLite, Microsoft SQL Server and Oracle Enterprise.
mx_banner.php should be installed on a web server that supports SSL and uses authentication, but I suppose for internal applications you could get by without those security layers.
Your web application can be written in any programming language that can query the maintenance events database and insert the event information into a div on your application’s post-login page.
If you host multiple applications that can experience unique maintenance windows, then just install an extra copy of mx_banner.php and the database schema for each application maintenance “zone.”
mx_banner.php is a minimal but non-trivial web application that is also good for learning PHP application programming. Also see the WordPress plugin I wrote in PHP on github.