Generally speaking, the best time to visit Thailand is from November to February when the northeast monsoon is blowing cool, dry air which serves as a respite from the heat. During this cool season, the temperature ranges from 18ºC to 32ºC in Bangkok, while in northern and northeast Thailand, temperatures can get quite cool with morning temperatures as low as 8º C to 12º C with the occasional 20º C day. Nights can be particularly chilly and at high altitudes the temperatures can and do drop below freezing.
The summer period, or hot and dry season, is from March to June. At this time temperatures in Bangkok average around 34º C, but can often reach 40º C with the humidity levels of 75%.
Try and avoid April, unless you plan to be permanently submerged in the ocean, because this is the hottest month across the country.
The monsoons finish when the wind direction changes, bringing dry weather from the northeast. At best this season can be described as unpredictable and not the constant downpour of rain like you would expect. The middle months of this season may hold particularly heavy rains for the north of the country.