The weather is best during the 'cool' season from November to February. It almost never rains and temperatures, while still warm, are cooler than the rest of the year.
The hot season from March to May sees temperatures soar to the high 30s and stay there. Any rain that falls at this time is a real blessing. Travelling can be arduous, but some of the best festivals are held during this time.
Isan is at its most beautiful during the rainy season (June to October) because the forests and rice paddies turn green and the waterfalls run wild. The rain can be a challenge, though.