Hua Hin Jazz Festival (31 May - 1 June)

Sorry I know very little about this - here's the Official Site and here's their facebook link
Enjoy it...

Candle Light Procession at Kwan Phayao (24 May)

The longer I live in Thailand the more that I realise that there are a great number of events such as this which I was totally unaware of - and still am. This was only because of a sideline in the Bangkok Post - so here is the text from them.

Thailand Travel Online 

... the Wien Tien Klang Nam ceremony of Kwan Phayao is something different, it breaks the from the tradition of the Wang Tien ceremony which usually held on the land.

Rayong Fruits Festival (23-29 May)

Not being much of a fruit lover i do not attend events such as this - BUT I do read that - 

Rayong is renowned for its abundance of tropical fruits and the best buys include rambutan, durian, mangosteen, longgong and zalacca. During the fruit season in the middle of May, the annual fair is held to provide a market for the locals from surrounding orchards to sell their fruits and other local products. The venue has been alternating between Rayong municipal town and Amphoe Klang. The fair has stalls selling local fruits and products. 

Colourful floats are decorated with fruits and flowers. Parades, musical entertainment, and dance processions wind through the streets of the provincial town. There are beauty pageants, fruit contests, local products and agricultural crop displays and exhibitions. Some of the major highlights are cultural performances and folk entertainment.

Text is from