5 December is His Majesty the King Bhumibol Adulyadej’s birthday, วันเฉลิมพระชนมพรรษา พระบาทสมเด็จพระเจ้าอยู่หัว in the Thai language. The King is the father of all Thai people, 5 December is also Father’s Day (“WAN PHO HENG CHAT” – วันพ่อแห่งชาติ).
Massive festivities are planned throughout Thailand, centering along the Chao Phraya river in Bangkok were 587 boats will sail down the river with over 8,000 floating lanterns and 30,000 people taking part in the procession.
Thai businesses and people pay tribute to the King and his birthday throughout the week leading up to 5 December. Pictures of the King surrounded by the Thai flag, the Royal flag and flowers are put up in front of businesses, inside small stores and in hotels and restaurants. An example:
Information about the festivities in Bangkok can be found here: http://www.nationmultimedia.com/2010/12/04/national/Kingdoms-greatest-show-30143793.html