I think the main thing - apart from echoing all Mr Red's points - is to get a clear picture of what kind of music and what kind of performance you want to put on, and build around that picture. Think what a really good (but achievable) night would be like, and then try and make sure that the first three or four nights are just like that. The message will get out far more effectively that way, both to the actual/potential audience and to actual/potential performers, than it would from any number of posters & emails.

For example, how much of a really good evening would be traditional? 100%, 90%, 50%, 10%? How much would be singers' own material? Would you be happy if half the night was taken up with Ewan MacColl songs? Or Dylan songs? Or Radiohead songs?

Would a really good evening involve a lot of audience participation? Or would the audience be there to listen attentively and clap at the end?

Would a really good evening involve a lot of virtuosic, professional-quality performances? Or would a really good evening be one where anyone felt they could rock up and have a go? If someone introduced a song by saying they hadn't played it in public before and putting the words on a music stand, would that make it less of a good evening?

Would a series of solo performers with acoustic guitars make for a really good evening? Or not? If every single performer adopted a fake American accent to sing with, would you be pleased/displeased/indifferent?

And so on. You can see the kind of question I'm getting at, and the kind of signals that the first few nights of a new club can send out.

Good luck with it. I think that if you build it they will come: there's real suppressed demand out there for music in performance, including traditional music. But you do need to decide just what it is you're building.