That information was embedded in a new Variety report about this year's Oscar race, in which The Last Jedi is mentioned as a possible contender. According to the report, "Disney will world-premiere" Episode VIII on December 8th, which would be in relative keeping with the traditional rollout plans for Lucasfilm.
Star Wars: The Force Awakens had its world premiere on December 14th, 2015, just four days before it hit theaters on December 18th; Rogue One: A Star Wars Story premiered on December 10, 2016, before officially hitting theaters on December 16th. Of course, those films were both positioned to take advantage of the holiday season movie rush.
If The Last Jedi has serious Oscar contention potential (for more than just the usual technical categories for blockbuster genre films), then getting it in front Academy voters and other critics going into those award season voting sessions in the first week of December is crucial.
Industry secret: by the midpoint of December, most critics' "Best of the year" lists are already locked into place, no matter which films they still haven't seen. That can heavily influence which films stay in the conversation during January's Golden Globes race - and stay in the conversation afterwards, as Oscar season approaches.
In any event, if Lucasfilm confirms this world premiere event date for Star Wars: The Last Jedi, the film will be off to the right kind of start to earn the industry's highest rewards. That is, you know, if director Rian Johnson has truly made a great film...