This holiday marks the birthday of the current queen of Thailand, Queen Sirikit.
If 12 August falls on a weekend, the holiday will be observed on the following Monday.
Sirikit was born on 12 August 1932. Sirkit met Rama IX in France in 1946 when he had already become ruler of Thailand. The royal couple were married on 28 April 1950, a week before the King's official coronation.
Across Thailand, buildings are decorated to honour their queen, with the most splendid are in Bangkok, where many buildings and streets are festooned with coloured lights, flowers and portraits of the queen.