Keep your eye posted to the Update from the North Pole section of My Merry Christmas and Santa Update.com -- both sites provide up-to-date information. For Santa it is a little different every year. I remember one year he didn't get going until around noon Eastern time and other year's he is dead on the nose at 12:01am on December 24h. It just depends what is going on.
There is an accurate countdown clock at this link