Disney has announced the opening date for Rose of the Resistance in both Disneyland and WDW, with the opening in DHS on December 5, followed by the Disneyland version opening on January 17, 2020.

Quote Originally Posted by Disney Parks & Resorts

... Star Wars: Rise of the Resistance will open Dec. 5 at Walt Disney World Resort. If this date sounds familiar, many of our fans will recognize it as Walt Disney’s birthday. What better way to honor the ultimate storyteller than by introducing the most immersive and advanced attraction ever imagined in a Disney Park.

When it opens, Star Wars: Rise of the Resistance will blur the lines between fantasy and reality and will put guests in the middle of a climactic battle between the First Order and the Resistance. Guests will be recruited to join Rey and General Organa at a secret base. Along the way, they will be captured by a First Order Star Destroyer. With the help of some heroes of the Resistance, they break out and must escape the Star Destroyer, protect the secret base, and stay one step ahead of Kylo Ren.

As soon as work is completed at Walt Disney World, Imagineers will head back to California to complete their mission at Disneyland Resort where Star Wars: Rise of the Resistance will open on Friday, Jan. 17.
Speculation is that an issue in getting the ride operational required some sort of construction rework at Disneyland. Work that likely hadn't been done at DHS yet. Resulting in the openings flipping from the expected schedule.

Steve