The only problem is don't you need light to see the cards? Or to read the piano music?
I am always so paranoid of having lit candles around kids (and clumsy people like myself)...
But I really like this idea. I would like the city to turn off the streetlights, too, but ours are automatic; they go on and off depending on the available natural light.
We have a "take back the night" night in August, and people turn their outdoor light on and go outside and walk and mingle.
It could be set up similarly...