With Flamenco shows you'll need to check specific performance times as they vary. If it's around the Ramblas area it'll be quite touristy so you may want to ask the concierge if he knows of anything else happening as well.
That's the best advice if you don't have any friends in the area. Have a talk with a concierge at any 5 star hotel and find what works best for you. I recall a great flamenco place just off Plaza Mayor, but I don't know the dates and times of the shows.