There will be a Highland Games programme listing all the meetings
for the year/season?-
Cowal Highland Gathering in Dunoon has a ceilidh tent so I expect some of the others will have similar?
In Inverness you will find MacGregor's Bar, it is a traditional music session venue owned by Bruce MacGregor of Blazing Fiddles.
There are a good few folk festivals pm around the country - maybe check the TMSA website (Traditional Music and Song Association) for listings?
Quite a few festivals are over a weekend....
Edinburgh Festival Fringe is on for I think 3 weeks in August (and maybe start of September?) BUT - Edinburgh will be jam packed with people and performers at many venues in and around the city - also accommodation will be hard to find!

There are also some week long festivals dedicated to fiddle or harp or bagpipes etc - often in smaller places like Blazin'in Beauly, Sabhal Mor Ostaig on Southern Sleat peninsula of Skye, other 'schools' can be found on Mull, some of Hebridean islands, Orkney, Shetland etc