User blog comment:Psypher/Young Justice: Legacy is complete. Less than a month until release./@comment-1038387-20131118205112/@comment-5188108-20131118222654

I'm guessing they're going for a simultaneous worldwide release on Steam and it's a timezone thing. It looks like it goes live at 23:00 on the 19th in the UK, which is the afternoon of the 19th in the US, and early on the 20th on mainland Europe.