Thailand has a lot of holidays, mostly related to Buddhism and the monarchy. Nobody celebrates all of them, except for banks, which seem to be closed a lot.
Makha Bucha (มาฆบูชา) - falls on the full moon in of the fourth Lunar month, which usually falls in February or March, and commemorates the spontaneous gathering of 1,250 people before the Buddha, which led to their ordination and subsequent enlightenment. At temples in Bangkok and throughout Thailand, Buddhists carry candles and walk around the main shrine three times in a clockwise direction.
During Chinese New Year (ตรุษจีน), Chinese Thais, who are numerous in Bangkok, celebrate by cleaning their houses and offering food to their ancestors. This is mainly a time of abundant feasting. Visit Bangkok's Chinatown or Yaowarat to fully embrace the festivity.
Songkran (สงกรานต์) - undoubtedly the most fun holiday - is the celebration of the Thai New Year, sometime in April (officially April 13th to 15th, but the date varies in some locations). What started off as polite ritual to wash away the sins of the prior year has evolved into the world's largest water fight, which lasts for three full days. Water pistols and Super Soakers are advised and are on sale everywhere. The best places to participate are Chiang Mai, the Khao San Road area in Bangkok and holiday resorts like Pattaya, Ko Samui and Phuket. Be advised that you will get very wet, this is not a spectator sport. In recent years, the water-throwing has been getting more and more unpleasant as people have started splashing iced water onto each other. It is advisable to wear dark clothing, as light colors may become transparent when wet.
Loy Krathong (ลอยกระทง) falls on the first full moon day in the twelveth month in Luna calendar, usually on November, when people head to rivers, lakes and even hotel swimming pools to float flower and candle-laden banana-leaf (or, these days, styrofoam) floats called krathong (กระทง). The krathong is meant as a thank you offering to the river goddess who gives life to the people. Thais also believe that this is a good time to float away your bad luck and many will place a few strand of hair or finger nail clippings in the krathong. According to tradition, if you make a wish when you set down your krathong and it floats out of sight before the candle burns out, your wish will come true. Some provinces have their own version of Loy Krathong, such as Sukhothai where a spectacular show takes place. To the North, Chiang Mai and Chiang Rai, have their own unique tradition of floating Kom or lit lanterns balloon. This sight can be breath-taking as the sky is suddenly filled with lights, rivaling the full moon.
Coronation Day (May 5) commemorates the crowning of the current King in 1950 (although his reign actually began on June 9 1946 - making him not only the longest-serving monarch in Thai history, but also the world's longest-serving current Head of State).
The King's Birthday (December 5) is the country's National Day and also celebrated as Father's Day, when Thais pay respect to and show their love for His Majesty the King. Buildings and homes are decorated with the King's flag (yellow with his insignia in the middle) and his portrait. Government buildings, as well as commercial buildings, are decorated with lights. In Old Bangkok (Rattanakosin) in particular, around the Royal Palace, you will see lavish light displays on trees, buildings, and the roads. The Queen's Birthday (August 12) is Mother's Day, and is celebrated similarly if with a little less pomp.