1. May National Labour Day
3. May Substitution for National Labour Day
5. May Coronation Day

King Bhumibol Adulyadej was crowned on 5. May 1950.

Phi Ta Khon Festival

Men masqueraded as spirits parade a Buddha image and tease bystanders while monks recite the ancient Buddhist tale of Prince Vessandorn, one of Buddha’s incarnations. His welcome back procession was grand enough to inspire the spirits to join the festivities.
Celebrated in Loei.

9. May 2010 Royal Ploughing Ceremony Day
28. May 2010 Visakha Bucha Day / Visakha Puja / Buddha Day

Also spelled Vesak, Wesak or Vaisakha and it is the name of this month in the Indian calendar. Celebration of Lord Buddhas birthday, enlightenment and death (attainment of Nirvana).