It's set to be released at Christmas.

There's going to be another Downton Abbey film.
The movie, due out at Christmas, will feature the original cast as well as newcomers including Laura Haddock and Dominic West.
Creator Julian Fellowes will write the film and it'll be directed by Simon Curtis.
We're thrilled to announce that Julian Fellowes and the entire Downton cast are back for #DowntonAbbey2, with Hugh Dancy, Laura Haddock, Nathalie Baye and Dominic West joining!
See the film in theaters this Christmas. pic.twitter.com/OHopFgzqiM
— Downton Abbey (@DowntonAbbey) April 19, 2021
Digital Spy reports that production started on Downton Abbey 2 last week, with a release date of December 22nd 2021 in mind.
It's currently unknown what plot or storyline the new film will follow.