The first time it runs the Application_Start event will execute in your global.asax file.
http://msdn.microsoft.com/en-us/library/vstudio/ms178473(v=vs.100).aspx[
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However, that is the first time someone visits the application after the App pool instance has been started.
To check for first time in the day you'll just have to record it in the db somewhere and you could use Session_Start in Global.asax to check to see if that user is the first for the day or not.