January/February
Chinese New Year, nationwide. Head for any Chinatown to see the vivid parades, firecrackers, and Lion Dances associated with this holiday. Things get most raucous in Bangkok's Yawarat district (Chinatown). It falls anytime from mid-January to mid-February, during which many businesses close for the week.
February
Flower Festival, Chiang Mai. When all of the north is in bloom, Chiang Mai springs to life with parades, floats decorated with flowers, and beauty contests. First weekend in the month.
March
King's Cup Annual Elephant Polo Tournament, used to be held in Hua Hin but now takes place near Chiang Rai in the Golden Triangle. Mahouts and madcap international polo players meet to battle it out in the mud in a hilarious but worthy week-long tournament that raises money for Thailand's main elephant charities. Entrance is free, and the cup happens during the last week of the month.
April
Pattaya Festival, Pattaya. Parades and fireworks accompany a food festival and lots of partying, during the first week of the month.
July
Koh Samui Regatta, Koh Samui. Late July brings yachtsmen and partygoers from all over to enjoy a week of fiercely competitive ocean races.
October
Vegetarian Festival, Phuket. In this bizarre religious ritual, devotees spear, pierce, and percolate themselves while in a trance. It's not for the fainthearted! It takes place the second week, and lasts for 9 days.
Naga Fireballs, Nong Khai, Isan. During this event, crowds gather along the river to witness red glowing balls rise from the waters; they're thought to be methane bubbles released from the riverbed. The second or third week of the month.
November
Elephant Roundup, Surin. Elephant parades and cultural performances take place during the third weekend of November.
December
King's Cup Regatta, Phuket. Global competitors race yachts in this exciting international event, which takes place the second week of December.