Thai lunar calendar

The Thai lunar calendar (, , , 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 irregular intervals.
Legal v. religious calendar.
The Thai solar calendar (, , ), 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.
Note that the Thai and the Chinese lunar calendars do not directly correspond. Thai Chinese likewise observe their Sabbaths and traditional Chinese holidays according to solar terms, two of which correspond to one lunar phase. These also move with respect to the solar calendar, and so it is common for Thai calendars to incorporate both Thai and Chinese lunar calendar-based events.
Mundane astrology also figures prominently in Thai culture, so modern Thai birth certificates include lunar calendar dates and the appropriate Thai Zodiacal animal year-name for Thai Hora (, ). The Thai Zodiac is similar to the Chinese, though the Dragon is replaced by the Naga (งูใหญ่), and in Northern Thailand the Pig is occasionally replaced with an Elephant.
Years.
To keep the years in sync with the seasons, Thai lunar years may add a day to the 7th month or repeat the 8th month. Therefore, years may have one of three lengths – 354, 355 or 384 days – yet retain a nominal length of twelve months.
New year.
The Thai lunar calendar does not mark the beginning of a new year when it starts a new 1-to-12 count, which occurs most frequently in December.
The Thai solar calendar determines a person's legal age and the dates of secular holidays, including the civil new year and the three days of the traditional Thai New Year, which begin the next Twelve-year animal cycle. Should the holidays fall on a weekend, it also accommodates these as well as some of the principal lunar festivals with a compensatory day off (, ).
Twelve-year animal cycle.
13 April of the solar calendar occasions the beginning of the traditional Thai New Year (Songkran) and is the day that a year assumes the name of the next animal in the twelve-year animal cycle; Thai Chinese communities may observed the name-change earlier in accordance with the Chinese New Year.
The Thai names of the months were borrowed from Khmer, which were in turn borrowed from an unknown Vietic language.
Months.
In the modern Thai calendar, months (, , , meaning "month" or "Lunation") are defined by lunar cycles. Successive months (or lunations) are numbered from 1 to 12 within the Thai year. As in other Buddhist calendars, these months have names that derive from Sanskrit, but for the most part are only known by Thai astrologers.
Two successive lunations take slightly more than 59 days. The Thai lunar calendar approximates this interval with "normal-month" pairs (, ) that are alternately 29 and 30 days long. 29-day "hollow months" (, , ) are odd-numbered (, , ); 30-day "full months" (, , ) are even-numbered (, , ).
To keep the beginning of the month in sync with the new moon, from time to time either the normally "hollow" Month 7 takes an extra day, or an extra "full" Month 8 follows a normal "full" Month 8.
Months 1 and 2 are named in archaic alternate numbers, with the remainder being named in modern numbers.
Months 1 – 6.
Month 1, "duean ai" (, ), begins the cycle of counting the months anew, most frequently in December, but does not signify the beginning of a new year. ai, an archaic word in Thai but not in other dialects, means "first". An odd-numbered hollow month, it is 29 days long.
Month 2, "duean yi", (, , from archaic ญี่ meaning "2") is an even-numbered full month.
Months 3–6, "duean 3–6", use the modern reading of Thai numerals, as do all remaining months. Months 3–6, alternate between 29-day hollow months and 30-day full months.
Month 7 and athikawan.
Month 7, "duean 7", a hollow month is normally 29 days long in years of 354 days, but adds an extra day ( ) when required for 355-day-long years (, ).
Month 8.
The eighth month, "duean 8", is a 30-day full month.
Month 8/8 "athikamat".
Athikamat (, )) is the extra month needed for a 384-day-long "pi athikamat" (extra-month year; , ). Month 8 repeats as เดือน ๘/๘ or Month 8/8, variously read as "duean paet thab paet" () or "duean paet lang" ()
Months 9 – 12.
Months 9–12, "duean 9–12", complete the lunar cycle.
Month divisions.
Months divide into two periods designated by whether they are waxing or waning:
Weeks.
A week is called "Sapda/Sappada" (, ). The term is defined by the Royal Institute Dictionary (RID) as a 7-day period beginning on Sunday and ending Saturday. When referring to lunations, however, it is the 7-, 8- or (rarely) 9-day interval between quartile lunar phases; that is, from one to the next.
Days.
While solar-calendar weekdays have names, lunar-calendar days number sequentially from 1 to 14 or 15 in two segments depending on whether the moon is waxing or waning. For example, "raem 15 kham duean 12 " means "Waning of the 15th Night of the 12th Lunar Month".
"Kham ", evening, is considered to be the evening of the common day that begins and ends at midnight, rather than of a day that begins and ends at dusk. Past practice may have been different.
Holidays regulated by the moon.
Buddhist Sabbaths, colloquially called , are the New, First-quarter, Full, and Third-quarter Moon-days. These are not normally days off (), except for butcher, barber, and beautician shops that observe the Eight Precepts. Annual holidays and seasonal festivals collectively are called .
Festivals or fairs are called ; these may be further styled as "traditional" and as , "rite" or "ceremony". The table shows the principal ones governed by the moon in yellow.
Work holidays prescribed by the government are called ; those regulated by the moon are red.
Weekends are normally days off; if a holiday normally observed by a day off falls on a weekend, the following Monday is a compensatory day off .
Notes:
Thai year vocabulary.
Thai orthography spells most native words phonetically, though there is no definitive system for
transcription into Roman letters. Here, native Thai words are immediately followed by a vocabulary entry in this pattern:
Example:
Sanskrit loan words follow different rules [the way English grammatical rules vary for words of Greek and Latin origin ('ph-' in 'phonetic' being pronounced /f/, for example.)] Entered below in order of first appearance, these vocabulary entries are in this pattern:
Literally means "self-made" or "self-done", or "cultured" in a modern usage (which implies the language of cultured persons); Sanskrit alphabet, language, writing; [presumed] compound of