I've asked a friend who lives in that areas. He writes, "Mid Wales events are covered in Broadsheep, a listings magazine covering jazz, world, folk and traditional music, as well as cinemas and galleries, and issued monthly from Nicola Humphreys, Ivy House, Broad Street, Presteigne, Powys LD8 2AF, tel: 01544 260051, e-mail: broadsheep@aol.com"Although it isn't 'folk', I think the Hay Festival of Literature is on in that period - pretty mega event these days with some major names. They had Bill Clinton last year ... but I think they were a bit mean when they refused to let the local dress shop sponsor his visit. Just 'cause it's called Monica's ...
Also, Gwyl Ifan (Feast of St. John's) is usually the third weekend in June, around the 18th? All the dance troups in Wales used to come down for a dance-around in Cardiff ... but I think they may concentrate on the festival at the Museum of Welsh Folk Life these days. That's become huge ... almost too huge, as you can hardly move there on these 'special' days, so not your cup of tea, I guess.
You'll miss the traditional season for the Gymanfa Ganu. They're usually held around Easter ... although they're not very common any more.
sian