The Thai lunar calendar (Thai: ปฏิทินจันทรคติ Patitin Chantarakati) (literally, Against-the-Sun Moon-Ways) is Thailand's version of the lunisolar Buddhist calendar used in the southeast Asian countries of Cambodia, Laos and Burma. Based on the third-century Surya Siddhanta Hindu calendar, these 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; Thai, Lao, and Cambodian versions do not add an extra day to years with an extra month.
Legal vs. religious
The Thai solar calendar, Patitin Suriyakati (Thai: ปฏิทินสุริยคติ), Thailand's version of the Gregorian calendar, replaced the Patitin Chantarakati in AD 1888 / 2431 BE for legal and commercial purposes. In both calendars, the four principal lunar phases determine Buddhist Sabbaths (uposatha), which are obligatory holy days for observant Buddhists. Significant days also include feast days. Thai Chinese likewise observe their Sabbaths and traditional Chinese holidays according to lunar phases. These move with respect to the solar calendar, so common Thai calendars incorporate Thai and Chinese calendar lunar dates for religious purposes.
Mundane astrology also figures prominently in Thai culture, resulting in modern Thai birth certificates that include lunar calendar dates and the appropriate Chinese calendar animal for Hora (astrology) (Thai: โหราศาสตร์ โหราสาต ho-ra-sat) and Chinese astrology.
To keep years in sync with the seasons, Thai lunar years may add a day to the 7th month, or may repeat the 8th, so may have one of three lengths—354, 355 or 384 days—yet retain a nominal length of twelve months.
- 354 day-long years consist of 12 normal months, and such a year is called a normal-month year (Thai: ปกติมาส (ปีปะกะติมาด Pee-pa-ga-ti-mat))
- 355 day-long years add an extra day to to the normally 29-day-long 7th month; such a year is called an extra-day year (Thai: ปีอธิกวาร (ปีอะทิกะวาน Pee-a-ti-ga-wan)).
- 384 day-long years repeat the 30-day-long 8th month, thus keeping the month count at 12. Nevertheless, a year of 384 days is called an extra-month year (Thai: ปีอธิกมาส (ปีอะทิกะมาด Pee-a-ti-ga-mat).
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 (See Month 1, below).
The Thai solar calendar determines a person's legal age, and secular holidays, including the civil new year as well as the three days of the traditional Thai New Year's feast. Should holidays fall on a weekend, it also accommodates these as well as some of the Principal lunar festivals with a compensatory day off Thai: วันชดเชย.
Wan Khao Phansah วันเข้าพรรษา counts as a new year for monks; note that 2 August 2004 was the compensatory day off วันชดเชย for a Wan Khao Phansah that fell on a Sunday. The Chinese calendar determines the day that a year assumes the name of the next animal in the twelve-year animal cycle.
Twelve-year animal cycle names
Months (Deuan (Thai: เดือน, meaning "month" or "Lunation") in the Thai calendar 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 would only be known by Thai astrologers (Prasert Na Nagara 1998:524, cited in Diller).
Two successive lunations take about 59 days. The Thai lunar calendar approximates this interval with normal-month (Thai: ปรกติมาสฅ ปกกะติมาด Pok-ga-ti-mat) pairs of 29 and 30 day months: 29 if an odd-numbered month (Thai: เดือนคี่ deuan kêe); 30 if an even-numbered month (Thai: เดือนคู่ deuan kôo). A 29-day lunation is called a hollow month (Thai: เดือนขาด deuan kàat); a 30-day lunation is called a full month (Thai: เดือนถ้วน deuan tûan). This is only marginally accurate, so at intervals either a normally hollow Month 7 takes an extra day, or an extra full Month 8 follows normal full Month 8. Note also that Months 1 and 2 are named in archaic alternate numbers, with the remainder being named in modern numbers.
The first month, Deuan Aai (Thai: เดือนอ้าย) begins the cycle of counting the months anew, most frequently in December, but does not signify the beginning of a new year. Aai, an archaic word in Thai but not in other dialects, means first-born (or eldest). An odd-numbered hollow month, it is 29 days long.
The second month, Deuan Yi, (Thai: เดือนยี่ from archaic ญี่ meaning 2) is an even-numbered full month that is 30 days long.
Months three though six, Deuan 3–6, use the modern way to read numbers as do all remaining months. Months 3–6, as they alternate odd and even are alternately 29-day hollow months or 30-day full months.
Month seven, Deuan 7, a hollow month, is normally 29 days long in years of 354 days, but adds an extra day (Thai: อธิกวาร (อะทิกะวาน A-ti-ga-wan)) when required for 355-day-long extra-day years (Thai: ปีอธิกวาร (ปีอะทิกะวาน Pee-a-ti-ga-wan).
The eighth month, Deuan 8, is a 30-day full month.
When an extra month (Thai: อธิกมาส (อะทิกะมาด a-ti-ga-mat)) is needed for a 384-day-long extra-month year (Thai: ปีอธิกมาส (ปีอะทิกะมาด Pee-a-ti-ga-mat)), Month 8 repeats as เดือน ๘/๘ Month 8/8, variously read as; Deuan Bad dap Bad —Month 8 slash 8, Deuan Bad Song Khang —Month 8 Side Two, Deuan Bad Song Hon —Month 8 Time Two in the Isan language. The last four months, Deuan 9–12, complete the lunar cycle.
Months divide into two periods designated by whether they are waxing or waning.
- Magha Puja Day - full moon day of the third lunar month
- Ashana Puja Day - on the fifteenth day of the waxing moon of the eighth lunar month
- Khao Phansa - on the first day of the waning moon of the eighth lunar month
- Loy Krathong - on the full moon night of the twelfth lunar month
- Phra Samut Chedi Temple Fair - on the fifth day of the waning moon in the eleventh lunar month
A week is Thai: สัปดาห์ or สัปดาหะ, pronounced สับ-ดา sàb-da, สับ-ปะ-ดา sàb-phà-daa, or สับ-ดา-หะ sàb-da-hà. From a Sanskrit word for "seven", it is now defined by the On-line Royal Institute Dictionary (ORID) 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 วันพระ wan prà to the next.
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. Examples:
- Kuen 1 Kham Deuan 1 ขึ้น ๑ ค่ำ เดือน ๑ - Waxing 1 Evening [of] Month 1
- Raem 15 Kham Deuan 12 แรม ๑๕ ค่ำ เดือน ๑๒ - Waning 15 Evening [of] Month 12.
Kham ค่ำ Evening, nowadays is generally taken as 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.