DANCING ON THE LEFT BANK - Paris
 

I have read that on certain days there is somewhere that becomes an impromptu dance thing, has anyone heard of this?!

This looks like the answer:
http://en.parisinfo.com/show_exhibition/rub7794.html

Yes, saw them from a Seine cruise two weeks ago. It was around 11pm, and still lots of people dancing there.

Thanks for that guys. Patrick, thats the one - thank you.

We too saw the dancing from our Seine cruise at about 10PM. It was a Monday night.