Phi Ta Khon is a famous mask festival held in the Thai province of Loei.
Below is taken from the wikipedia to give you further information on the festival.
Phi Ta Khon (also spelled Pee Ta Khon (Thai: ผีตาโขน), sometimes known as Ghost Festival), is the most common name for a group of festivals held in Dan Sai, Loei province, Isan, Thailand. The events take place over three days some time between March and July, the dates being selected annually by the town’s mediums.
The whole event is called Bun Luang, part of a Buddhist merit-making holiday also known as Bun Phawet.
The first day is the Ghost Festival itself, also called Wan Ruam (assembly day). The town’s residents invite protection from Phra U-pakut, the spirit of the Mun river. They then hold a series of games and take part in a procession wearing masks made of rice husks or coconut leaves with hats made from rice steamers, plus patchwork clothing, They also wear bells and wave wooden phalluses.
The origins of this part of the festival are traditionally ascribed to a story of the Vessantara Jataka in which the Buddha in one of his past lives as a prince made a long journey and was presumed dead. The celebrations on his return were so raucous as to wake the dead.
The second day of the festival incorporates elements of the Rocket Festival, plus costume and dance contests and more parades.
On the third and final day, the villagers listen to sermons from Buddhist monks.
Anyways, saw this guy on the streets of Bangkok dressed in full costume.
have yet to visit the festival, but it is on the to do list before we leave Thailand.