Actually, because 666 is the mark of the Beast, 3:33 is considered the peak of the darkest hour before the dawn. It's the time when evil has the best opening to the world of the living. I don't go for all that crap, but if you consider that they get a lot of negative activity, it makes sense. There's activity at any hour, but during that time, more negative activity can occur, supposedly. Also, it's harder to film spirit activity during the day because of the movement of the sun and shadows, etc. (as they proved in another episode of Ghost Adventures). So, it may or may not be true that there is more activity during dead time, but it does seem like there is.