The TMSA Calendar does show SOME, but not ALL Scottish festival: since they started charging for entries in it a few years back, quite a few festivals stopped sending their details for publication.
April: Glenfarg Folk Feast, now based in Kinross, is a great weekend.
Girvan has been mentioned: S Ayrshire, always on the May Bank Holiday weekend. There have been good singing sessions in the Westcliffe Hotel or Catholic Hall, usually after the evening concerts are over.
Moniave, also in SW Scotland is usually the weekend after.
Meanwhile in Edinburgh, there is Tradfest, which runs for about 10 days in early May.
The Fifesing is still going, but has had to move around: last year it was in both Freuchie and Falkland, but a bus was laid on to get people between venues.
To be continued!