It might be necessary to ask the people running a pub or a bar or whatever if it'd be all right to have a session, but I can't see how the people running a festival would have any authority to stop it happening; though as Nancy King reported there, some organisers can get a bit bossy about stuff that doesn't concern them. The thing about fringes is that they are not under the control of the organisers.

Of course different places have different rules and regulations and licences which can get in the way of that kind of things happening, as we in England know only too well, with a Licensing Law that might be designed to stamp out informal music-making.