First New Year's Day of Siam's adopted solar calendar

Background. The Thai solar calendar was adopted by King Chulalongkorn (Rama V) in 1888 CE as the Siamese version of the Gregorian calendar, replacing the Thai lunar calendar as the legal Thai calendar. The lunar calendar remained in use, especially for traditional and religious events. Chulalongkorn's reign from 1868 until his death in 1910 was characterised by the modernisation of Siam and governmental and social reforms, though the supplied passages do not state a direct causal link between those reforms and the calendar adoption.

Epoch and Reckoning. In 1889, King Chulalongkorn decreed a change in vulgar reckoning to the Rattanakosin Era. The epoch for Year 1 was established as 6 April 1782, coinciding with the accession of Rama I, the foundation of the Chakri dynasty, and the founding of Bangkok as the capital. This date served as the reference point for calculating years in the new era, with specific rules for converting to the Common Era.

New Year Observance. The first New Year's Day of the adopted solar calendar falls within the event scope for 1888. The specific date of 6 April 1888, the coincidence with the 106th anniversary of Bangkok's founding, and the Grand Palace location derive from a legacy row treated as a discovery hint, not from the supplied passages, which confirm only the 1888 adoption and the 6 April 1782 epoch. The passages therefore support the observance at year-level precision only.