The best time to travel to Thailand is late November to the end of April. This is the so called “dry season”, but don't be fooled by the name, you are in the tropics, so you always have to expect some outbreak of rain from time to time. There is one exception to every rule, in this case it's name is Koh Samui and the neighboring islands of Koh Phangan and Koh Tao; best to be visited in the European summer months from March to early October.

Temperatures in Thailand are high throughout the year, with the slight exception of the mountainous north, where “off season” and in the night or early morning hours, you might want to wear some slightly warmer clothes.

To give you an orientation, here are some climate charts for certain regions.