The game has one release date: May 19th.
May 15th is the date Premium Edition players can start playing due to a bonus: "four days' early access". This is standard convention in the gaming industry.
Some players, such as those in the media and some influencers, will be able to play much, much sooner than that, but we don't say that's the release date either, because it isn't.
It's also extremely likely that versions made available to limited player numbers prior to May 19th will be subject to a "Day One" patch, which will be part of the public build on - you guessed it - May 19th.