When is Session_End() called in ASP.NET MVC?

I want to sign out a user when his session times out. So used following code in Global.asax:

protected void Session_End(object sender, EventArgs e)
{
    FormsAuthentication.SignOut();
}

But seems session_end never fires. Any idea how can fix it? 

Session_end only fires in case of the Inproc session mode and not the Outproc session mode. In your web.config you need to have the sessionState element as a child of the element

<configuration>
     <system.web>
          <sessionState mode="InProc" timeout="2" />
          .....
     </system.web>
</configuration>

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.