Thailand has tropical clima and three main seasons.

From July to November is the rainy season, it's slightly cooler, although still hot by most standards, and it's a lot wetter.
Rainy Season: The southwest monsoon comes from the Indian Ocean with rain-laden clouds, from about June to October. Not every day rains so there is still plenty of sunshine. The humidity makes it feel hot and sticky.

December to February is the cool season most favored by tourists, the weather then is still quite hot, but not nearly so humid. In the north though, temperatures can drop to less than 10 degrees centigrade at night.
Cool Season: The northeast monsoon from central Asia usually blows from November to February, bringing relatively cool, dry conditions to Thailand.This is the best time to visit Thailand.

March to May, the land heats up, it is the hot season.  Eventually the high pressure over the Indian Ocean moves inland, and the monsoon cycle begins again. This is the hottest time of the year so the school closes for a two month long break.
Hot season: Between the two monsoons, provide a distinct, and fairly predictable, set of climate changes: Locals seek the shade more often than normal, air conditioners work overtime, and the unwise tourists sizzle and burn in the sun.
 

Bangkok
Ú Rainy season: May to October, max.rainfall in September
☼ Best season: November to February ca. 30°C.

Khon Kaen
Ú Rainy season: May to November, max. rainfall in July
☼ Best season: December to April ca. 28°C.

Phuket
Ú Rainy season: May to October, max.rainfall in September
☼ Best season: November to April ca. 33°C.

Chiang Mai
Ú Rainy season: May to October max. rainfall in July to September
☼ Best season: November to April ca. 28°C.

Ko Samui
Ú Rainy season: September - middle December
☼ Best season: middle December - April ca. 28° - 31°
☼ Peak season August ca. 30°

Here to convert “°Celsius” into ”°Fahrenheit”!