Visit Thailand preferably between February and November, the high season and the time of year when it rains less and is generally less hot. Except for climatic anomalies, April, June and September are valid months with fewer tourists, relatively lower prices and strong but quick showers in the late afternoon that refresh the atmosphere. October is usually the wettest month.
In case you are planning to explore only the northern mountainous provinces there is no big disadvantage if you choose the warmer season or even the beginning of the rainy season, in June and July.
The hottest season coincides with the months of March and May when temperatures rise almost every day to 40ºC, in the center and northeast of the country. This is a good time to spend time on the southern Thai beaches.