Winter (November to January) is the time to head to Northern Thailand, when daytime temperatures at the higher elevations are a relatively comfortable 20°C to 23°C. Nighttime temperatures can, in some places, dip perilously close to freezing.
From March to May, the hottest time of year, daytime temperatures climb close to 40°C and smoke from slash-and-burn agriculture can fill the skies.
The rainy season (June to October) should generally be avoided if you plan to do any hiking.