You may set some Session data in Session_Start. Without this, Session_End will not be fired.
Also another very important thing to note here is that if you do not save anything in the session the Session_End event will not fire. There must be something saved in the session atleast once for the Session_End event to fire. This also means that if you save something in the session in the first request and abandon the session in the same request the Sesison_End event will not fire, as there was nothing saved in the session ever.