When Session starts?

 Hi every one, this is a simple but difficult question....
I have login page, button click, if the user is valid I am writting,
FormsAuthentication
.RedirectFromLoginPage(Session["UserID"].ToString(), false);
 
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so
//button click
// validate user
// if validated RedirectFromLoginPage
// if failed show error message
 
My question is when is a session starts?
 when i request login ...