"Fringe" events are the casual or impromptu gatherings created by the festival-goers, as distinct from the main events where you pay to see performers.

The distinction that the OP was making is, I think, that "open Mic" sessions are pushing out the session-players. This is a distinction that may not make much sense in the US, but here in the UK, "Open Mic" events are usually run in a bar where singers/musicians get up one at a time, often with a microphone, to do a couple of songs. Sessions, on the other hand, are musical gatherings where everyone joins in and plays together, at least for most of the time.

And I recognise the problem. It depends a lot on the attitude of the local pubs, and on the festival organisers, who can help by getting someone to set up and run a session. If this isn't done, pubs may decide that what is needed is an "Open Mic" event, which pushes out the session players. Other pubs may be trying to concentrate on serving meals, or giving TV coverage of footbal matches, and it can be quite difficlut to find a pub with space for a session.

That's why musicians at a festival can end up roaming the streets looking for a space to play. It's finding a suitable and welcoming venue that's the problem. At least, I think that's what's being said. Well it is now, 'cos I've said it.

Marje