It's kind of funny, considering what a resounding success the new Star Wars films have been, to think of all of the times things have shifted for the relaunched Lucasfilm series. If you follow the history of the films that have gone into production since Disney purchased Lucasfilm in 2012, you'll see that the company has no problem moving things around.
While initial rumors for Star Wars: The Force Awakens found the film arriving in "Summer of 2015" since, historically, Star Wars has been a summer franchise, that movie ended up arriving in December of '15 after the studio threw out Michael Arndt's script and had J.J. Abrams and Lawrence Kasdan rewrite it. Then Rogue One: A Star Wars Story followed the precedent established by The Force Awakens by coming out in December of 2016, and it was going to be followed-up five months later by Star Wars: Episode VIII in May of 2017. Episode VIII got its release date officially shifted to December of '17 in January of '16.
And now, once again, a Star Wars film that was once spoken of as "arriving in May" of 2018 is now going to get pushed into December of that year.
It would appear that, despite their best intentions to keep the "Star Wars in the summer" tradition alive, the new Lucasfilm is just going to re-brand December as Star Wars Month.
The latest scuttlebutt is that 2018's Han Solo: A Star Wars Story has quietly moved from May to December. It comes courtesy of Making Star Wars, which claims to have sources that are telling them that Han Solo will drop on December 13, 2018.
The writing was on the wall for this shift in release dates if you paid attention to the casting announcements for Donald Glover and Emilia Clarke. When those actors boarded the Han Solo movie, suddenly you started seeing wording like "arriving in 2018" instead of the once-firm May release that had been attached to the project ever since it was announced. So Lucasfilm has known for some time that Solo wouldn't be arriving in May.
None of this should be cause for concern, of course, as it would appear that Lucasfilm is simply taking advantage of the fact that the two Star Wars have done so well, and with Episode VIII coming out this December, audiences have successfully been trained to think they'll be visiting that galaxy far, far away every Holiday Season.
One interesting subplot in all this is the fact that the rumored reason Lucasfilm took so long pulling the trigger on moving Han Solo to December was because of James Cameron's Avatar sequels. For a while, the sequels to the #1 movie of all time globally, were expected to arrive every December from 2018-2020. But with Cameron being non-committal about being tied down to release dates, and Sigourney Weaver recently getting vague about whether or not the first Avatar sequel will actually arrive in December of '18, it looks like Lucasfilm is just going to pounce on that release window.