iRule to display one time warning message

If I may add, the trick is going to be how you track when and if a user has seen the message. HTTP is stateless in nature, so you need a mechanism to detect that the user has seen it at some point in that day. Some options include a long-lived session table entry on the LTM that stores the client's IP or user logon information, or more basically, a persistent cookie in the browser that expires daily. Here's an example:

when HTTP_REQUEST {
    if { not ( [HTTP::cookie exists BANNERMESSAGE] ) } {
        set expires [clock format [clock seconds] -format "%A, %d-%b-%Y 20:00:00"]
        HTTP::respond 200 content "html banner content" "Set-Cookie" "BANNERMESSAGE=1; path=/; expires=$expires"
    }
}

If the BANNERMESSAGE cookie doesn't exist in the request, the iRule generates the HTML banner page and inserts a persistent (file-based) cookie that expires at 8pm that day. You'd probably want a link or button in the HTML content that lets the user acknowledge the message and redirects back to the application.