The Thai lunar calendar, 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 irregular intervals.

A lunisolar calendar is a calendar in many cultures, incorporating lunar calendars and solar calendars. The date of lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the Earth's sky. If the sidereal year is used instead of the solar year, then the calendar will predict the constellation near which the full moon may occur. As with all calendars which divide the year into months there is an additional requirement that the year have a whole number of months. In some case ordinary years consist of twelve months but every second or third year is an embolismic year, which adds a thirteenth intercalary, embolismic, or leap month.