Thai lunar calendar

The Thai lunar calendar (Thai: ปฏิทินจันทรคติ, rtgs: patithin chanthrakhati, pronounced [pà.tì.tʰīn tɕān.tʰrá.kʰā.tìʔ], literally, Specific days according to lunar norms), or Tai calendar, is a lunisolar Buddhist calendar. It is used for calculating lunar-regulated holy days. Based on the SuriyaYatra, with likely influence from the traditional Hindu Surya Siddhanta, it has its own unique structure that does not require the Surya Siddhanta to calculate. Lunisolar calendars combine lunar and solar calendars for a nominal year of 12 months. An extra day or an extra 30-day month is intercalated at regular intervals.

The Thai solar calendar (Thai: ปฏิทินสุริยคติ, rtgs: patithin suriyakhati,  [pà.tì.tʰin sù.rī.já.kʰā.tìʔ]), Thailand's version of the Gregorian calendar, replaced the patithin chanthrakhati in AD 1888 / 2431 BE for legal and commercial purposes. In both calendars, the four principal lunar phases determine Buddhist Sabbaths (Uposatha), obligatory holy days for observant Buddhists. Significant days also include feast days.

...
Wikipedia