The festival occurs on the full moon in November (same day as the full moon party which happens down in the south). Everyone is supposed to have a lantern. The lanterns are made of this thin paper material and are attached to a small ring of wood (bamboo I think) and when lit, it creates this little hot air balloon/lantern that floats up. Each lantern is supposed to represent a wish that the person who releases the lantern makes to the Buddha.
After the Monks give their blessing they give an announcement telling everyone to light their lanterns, make a wish and then everyone standing in this large field lets their lanterns go. It looks a little something like this:
Or if you're in the middle of the field (which I was not) it would look something like this:
It was absolutely beautiful. The lanterns continued to go up into the air well into the night. Some were caught in trees and some just fell, but the majority floated away. Thousands of wishes ascending into the night sky.