IMDB says January 19 but I've heard everything from December 2018 to June 2019.
shareThe Univeral Pictures site also lists 1/18/19, so I think that confirms it. I’m looking forward to it.
https://www.universalpictures.com/movies/glass
A number of sites (Fandango, Rotten Tomatoes, Wiki, Universal Pictures, etc) have its release date listed as January 18. The date on the Universal Pictures site is perhaps all the confirmation you need.
There could be an early special screening for audiences, but you can bet January 18 (17 if you count opening day) is when it’s likely going to drop in theaters. The trailers themselves say it’s coming this January.