Session_End is Fired when a user's session times out.
When ever the Webserver does not get any response for 20 min then by default the Session_end fires
automatically.
Another important thing to mention in this topic here is the Session_end only fires
incase of the Inproc session mode and not the Outproc session mode.
We can use the Session.Abandon to force this.
Remember the Session_End event will not fire in the case where the user closes the
browser. HTTP is a stateless protocol. So there is no way for the server to
understand that the browser has been closed.