This isn’t coming from Disney or Lucasfilm directly, so for now at least, it needs to be taken as a rumor, but the often reliable fan site Making Star Wars is reporting that the studio is looking to shift Han Solo: A Star Wars Story to a December release date.
Even though Han Solo: A Star Wars Story would still probably do very well (it is a Star Wars movie, after all), it could be limited by some stiff competition.
That should give them a lot of confidence as they head towards production on the young Han Solo movie.
At this time, Disney and Lucasfilm are currently seeking extras for Han Solo as it begins shooting in early 2017.