Thai fruits including mangoes, managosteens, durians, pineapple, watermelons, papaya, rambutans, longans, lychees, tamarinds, pomegranates, palm fruits, oranges, pomeloes, jackfruits and more than 20 kinds of bananas are available all year round.

From January to April, grapes, jackfruits, java apples, tangerines, watermelons and pomegranates are in season. Next, come mangoes, lychees, pineapples, durians and managosteens.

From July, longans will ripen, and also langsats, jujubes, passionfruits, pomeloes, rambutans, suger apples and again tangerines, grapes, watermelons, bananas, coconuts, guavas and papayas are available.

Some harvests are celebarted in style, with colourful festivals, sometimes featuring a pageant of local beauties.

Source:
http://www.modernthailand.com/eng/travel/thai-food/thai-fruits.htm
Photos courtesy of:
http://www.yummytaste.com/ingeneral/fruits.htm