Loy Katong is essentially the Thai Thanksgiving and is celebrated during the full moon in November (if it happens to be a month with two full moons, I guess it happens on the first one). This is when people give thanks to the rivers for the water and way of life they provide.
There are several parts to the celebration, but the big one involves floating lotus-shaped banana leaf constructions with candles and incense on the river, to be done with loved ones ("loy" means float and "katong" is the lotus-craft). Many people make their own katongs, and they range from pretty simple (like the one I made) to ridiculously elaborate.
The other parts of the celebration are the sending off of kom-fei (paper hot air balloons), which looks awesome when everyone does it together and you have a full moon for a back drop; a parade where each village in the area decorates a truck and has a girl (or girls) sit on it who participate in the ensuing beauty contest; and the parade ends at a festival with drinking and singing and dancing and the afore-mentioned beauty contest.
Just thought I'd do a quick little "cultural sharing" post to fulfill that third goal of the Peace Corps mission statement (to share the cultures of other nations with Americans).