His Majesty King Bhumibol Adulyadej was crowned as Rama IX, the 9th king of the Chakri dynasty, on 5th May 1950. The anniversary of this day has been observed as a public holiday ever since.
In former times, the accession to the throne of a Siamese sovereign was not celebrated publicly. Only a simple ceremony was held by the court officials, in which the royal title and articles of royal use were presented to the king.
After King Mongkut (Rama IV) ascended the throne and performed the traditional ceremony on 6 April 1851, he issued an edict saying that the coronation of a monarch was regarded as an auspcious occasion in all countires ruled by a sovereign and was joyfully celebrated by the people of the whole country. However, this had never been done in Siam. The king, therefore, ordered that his coronation be celebrated by inviting Buddhist monks to the Grand Palace to chant sutras on the 13th day of the 6th moon and again to be entertained at a feast the next day. This was the first time the enthronement of a Thai king was celebrated.
In the present reign, Coronation Day is observed by a series of religious rites lasting for three days. On the first day, the 3rd of May, there is a Buddhist ceremony at Amarindra Vinichai Hall in the Grand Palace dedicated to the ancestors of the Chakri House, in which scriptures are chanted and a sermon is delivered by a high monk. On the second day, the ceremony consists of two parts – a Brahmanic one and a Buddhist one. The third day is Coronation Day. A feast is given to Buddhist monks and the king is dressed in full regalia. At noon, the Royal Thai Army and the Royal Thai Navy each fire a salute of 21 guns. Later this day decorations are bestowed on officials and civilians who have done meritorious services to the state and society.
Source: Essays on Thailand by Thanapol Chadchaidee