To me, its that moment when the muster drill ends and you realize that NOW you are on vacation and are free to do whatever you want. There is really no other feeling like it!
While on a cruise, during the first few days, time seems to pass quite slowly. It seems like when I wake up on the ship the first morning that I have already had such a great time and experience that I can't believe that I still have six days left. As the hours pass, that feeling intensifies until about day 5 when I begin to realize that the trip is winding down. But during those first four days, I am in heaven.