On Koh Phangan, like elsewhere, the Full Moon Party follows the moon where it not for the Buddha day, which often falls around full moon.  When the date coincide the party is simply postponed to what usually amounts to the next day. On Buddha-Day it is regularly not allowed by law to sell alcohol or celebrate with loud music. Some of the Buddhist holidays are stricter with the enforcement than others, and it is very difficult to find out ahead of time which of the Buddha days is holiest, so beware.