Last year, I arrived on 26 June and had a pleasant stay there. Before going, I had asked ahead and learned they where indeed already open for business.
According to their web site, in 2013 they open on 15 June, so I reckon that mid June is normal.
Thanks for info! I put it on Schesaplana page. By the way - if you want to have editing or admin rights I'm happy to grant them to you.