I know I am grasping at straws here, but on some other forum (honestly can't remember where) I remember reading someone suggesting that the release day will be conditioned by Frontier's financial year. So I went to check on their 2016 financial report (http://ar2016.frontier.co.uk/assets/pdf/frontier-developments-2016-annual-report-and-accounts.pdf) and indeed their financial years end on May 31st. Considering that the 2016 report makes a big deal of the XBOX launch of ED (released in October 2015), one could speculate that they might want to feature the revenues from the PS4 launch on ther 2017 report. To make it happen, however, I guess one would need at the very least a couple of weeks of data to include before closing the report. So well, my wild speculation (and maybe hope?) is that we'll see the game launched sometime between the 1st and the 15th of May at the latest. Then again, they might manage to get everything ready before that!