The best time to visit Thailand is from November to March. However, it really depends which part of Thailand you are visiting. The south of Thailand is less hot than the northern part especially from April to June. So if you have to visit Thailand during that time, its better to go to southern cities such as Phuket.

The best time to visit the north is from mid November to December, and February However, it is not too bad to visit northern Thailand cities such as Chiang Main from March to May if you want to avoid the crowd.

July to October are the wettest season in Thailand with lots rain, but the weather is bit cooler, and since it is not the peak of the tourist season, you may get some good bargains for hotels and flights.Red block indicates high season
Green block indicates low season
Yellow block indicates sun
Blue block indicates rain