The original plan for the new Star Wars trilogy was to release the movies every other summer but that idea was forced to be adjusted when it was clear that Star Wars: The Force Awakens wouldn't be able to meet the deadline of essentially completing a CGI heavy film in just 18 months. Now it appears what was a decision forced by circumstance has become the norm for upcoming Star Wars films as Star Wars Episode VIII was bumped 7 months to December 15, 2017 instead of May 26, 2017. Star Wars Rogue One is the next film set for release on December 16, 2016. The decision was driven by the near $2 billion box office take The Force Awakens has enjoyed and likely would not have reaped in the highly competitive summer movie season.
Personally I have long been confused on why the studios don't stake out the December corridor more often. Its no accident that the three top movies of all time were released during the holidays. The competition is much lighter (unless Oscar bait type films) as there are traditionally few tent pole level movies released then. If think about this last month, if you were in the mood for an action film, or a science fiction film, or just a fun film (that wasn't Oscar bait), your option was Star Wars or nothing. And this has been typical for years now. Assuming Disney plans on staking out mid-December for the next five years for each of their Star Wars films it will be interesting to see if the other studios respond by possibly moving what would have been summer films into the holiday time frame to try to soak up some of the Star Wars bucks.