In MVC Global.asax file, we can see the Application_Start where this event fires only for once. But the session is not yet active/available here. So my question is, is there any event in Global.asax file that fired only for once and session is available also?
The reason I ask this is because, I use ExpandoObject
e.g.:

    public static dynamic Data
    {
        get
        {
            #region FAILSAFE
            if (HttpContext.Current.Session[datakey] == null)
            {
                HttpContext.Current.Session[datakey] = new ExpandoObject();
            }
            #endregion

            return (ExpandoObject)HttpContext.Current.Session[datakey];
        }
    }  

I want to initialize all of my ExpandoObject at once with the value of null:

MyExpando.Data.UserInformation = null;  
MyExpando.Data.FolderInformation = null;

That's why I'm looking for an event that only fired once.