Now my personal encounter with this ritual is something else forget everything you read in the link above, when its nice and sunny outside everyone is a fair game.
Picture a bus stop right in front of the building entrance. Everyone enters the bus and just as the doors close 2 guys run out with buckets of water and poor it on who eve is unlucky enough to stand at the door. Another instance that i remember that happened in school. Our teacher had to leave the class for a moment. While she was out bunch of students rushed in with water bottles and lets just say that girls got the most of it. The teacher was horrified to see what happened while she was out, as if the monsoon went thru the classroom, the water was dripping of the sealing, window curtains, walls.
It is fun to the extent and even taken to the extreme no one gets hurt.
I believe Wet Monday would not work here in the states, simply because some one would get hurt or worse shot out of lack of better jugement.