
You can enter your guess and the system will hint if you are too early or late. According to the page Microsoft will probably release Windows Vista on December 1 or November 30.
According to a web-developer mate of mine, the videos are stored on a Microsoft server in the otherwise neutral country of Switzerland and are stored in one of two sub-directories.
One of the sub-directories is called 'early' and ends in November 30 and the one called 'late' starts on December 1.