Doctor Who Series 12: Everything We Know So Far
1. It Will Start Airing In "Very Early" 2020
At the same time they made the announcement about the main cast and Chibnall returning, the BBC also confirmed that Series 12 would start airing in "early" 2020, although they didn't provide a specific month or date beyond that vague window.
However, a slightly more detailed start date arrived later, courtesy of Tony Hall, the Director General of the BBC. While attending a special screening of Resolution, Hall stated that the Thirteenth Doctor's next collection of adventures would start hitting screens "very early in 2020".
Now yes, that's also quite vague, but "very early" sounds like a window of around January to March, while just "early" could extend that window up to April or May. The earliest premiere date for a series of NuWho is March (Series 1 and Series 3 both started in that month), so that also wouldn't be out of the question here.
The majority of Series 12's episodes will be well on their way to completion at this point too, so don't be surprised if we get to see them in late January or early February.