There's the Ioan Rhagfyr Session which is part of Sesiwn Fawr (http://www.sesiwnfawr.co) , on that weekend. Blurb from Facebook:   

20 July · 19:30 - 23:30

Gwesty Gwernan, Islaw'r Dref, Cader Idris, Dolgellau, LL40 1TL, Wales

A traditional music session that meets monthly (usually 3rd Wednesdays) in or near Dolgellau.

The name of the session is inspired by Ioan Rhagfyr (1740 -1821), who was born in and lived in Dolgellau. He was one of the first known players of the flute in Wales. He also composed dance music for the flute.

The session, whilst it mainly reflects the inherited music of its locality and concentrates on Welsh traditional tunes, is open to all kinds of music and acoustic musicians such as singers, fiddlers, accordionists, pipers, banjo players, harpers and so on. It especially seems to attract flute players including Ceri Rhys Matthews, Jem Hammond, Chris Swan, Tom Scott, Nigel Hardy, Diarmuid Johnson, Hefin Wyn Jones, Rowenna Williams, Chris Bursnall and Maria Hayes (not all at once!). We often have 15-20 (or more) musicians attending from a wide area (Cardiganshire to Wrexham, Llanidloes to Caernarfon) - many travel 50 miles or more!

The main venue (when available) is the Gwesty Gwernan (http://www.gwernan.co.uk/), about 2 miles out of Dolgellau, but occasionally we have to find other venues in the area.

Geraint at the Gwernan is able to host us this month :-).
Also, we are listed on the Sesiwn Fawr Dolgellau programme as the 20th falls in the week of the festival.