The launch date of Star Wars: Episode VIII has been pushed out by six months, with the previously anticipated summer 2017 release of the next main line sequel put back until December. After all, it worked pretty well for Episode VII.
No reason for the delay has been offered by Disney, so we have to guess. Perhaps it's been delayed so the scriptwriters can address the trillions of internet plot-hole posts? The actor that played Chewbacca shaved all his hair off for another role and it won't have grown back in time? It's taking longer than previously thought to make an entirely CG Harrison Ford? Carrie Fisher needs more time to gain weight and age more to piss off people on the internet?
It's probably more to do with how much money it made in a very short period of time, as bored man-children and actual children scheduled numerous repeat viewings over the Christmas period. After all, even if you don't like Star Wars very much, the offer of sitting in a warm room watching a film and eating sweets for two and a half hours is much harder to turn down in December than July.
Not to mention the toy sales, which nearly equalled the take of the film itself.
To add further detail to your struggle to pre-book tickets for a film 23 months in advance, it's been noticed that Episode VIII appears to have one of those comedy production names movie studios often use to mask what they're doing. A casting agency is looking for people to be in a forthcoming production known as "Space Bear" -- the working title for the next Star Wars proper. [Independent, The Register]