There is a lot of confusion in developers that when Session_End event is fired. I wrote a small article on this issue.
Here is few lines from the article:
Session End event is fired only in the In-Proc Session mode at the end of the request in which Session.Abandon() is called OR when the Session time expires. Session End will not be fired when you close the browser. Let's make a small test in the Global.asax file so that we will know that Session.Abandon is being called.
Check out the complete article at:
When Session_End is fired?