It is opened in May and run every Friday until the end of September. A seasonal card - a green card - is valid for the whole summer and you can buy it for 240kr. There is Tango on Fridays and salsa, foxtrot or bugg the other evenings and all this is available with the same card same price.
Some pictures from last Friday, this year!
On this video you find the place from a different angle and in the beginning you get the directions to Grönalund tivoli by tram and in the end by the boat. The last one is a good summer option and easy to reach from the pieer at Slussen. (the tram stop has been moved further down after this video was completed - just a some meeters more to walk in same direction, same street)