Depending on transport, you can travel to some even more northerly parts of Scotland in just a few hours.
OK, some of the islands and outlying rural areas are less accessible but it can be sometimes as long to travel to some of the smaller towns further South.

Anyway, here are some of my favourites.

Niel Gow Fiddle Festival at Dunkeld 20-22nd March

The Edinburgh Harp Festival 3-8th April.   Actually, there's a lot more than harp(Fine an instrument as that is) with lineups featuring other instruments and song. Also late night sessions where a variety of music, song etc is welcome

Glenfarg Folk Feast 3-5th Sadly clashes with Harp Fest again... :-(

Girvan festival 3-5th May
Moniaive Festival 10-12th May
Keith Festival      12-14th June
Newcastleton       Usally first weekend in July. TBC

AS Kenny says, the very wonderful STonehaven Festival 10-12th July.

There's also a new one at Newton Stewart in July. Very good, apparently but it clashes with Stonehaven.

A new kid on the block is The Ardersier Folk Festival 21-23rd August.

As Jack says, the fiddle festival is good and not that "samey" if you are a fiddler.

There's also a Guitar and mandolin fest in Edinburgh this year at Merchiston the week before the Harp Fest.

If you wish to visit Edinburgh during The Fringes, there's lots going on although the folky acts are generally nothing too exciting and often feature certain musicians who probably wouldn't secure a decent booking at any other time. :-))

There's many more, of course. Linlithgow is nice and friendly in September and Dunbar is fine if you like things really "low key". :-)