Season 18 ends on the 21st of March 1981, and Season 19 begins on the 4th of January 1982. Yes, Doctor Who has shifted back to its old season slot and now begins at the start of the year once more. But with some differences.
At 41 weeks between Seasons 18 and 19, Doctor Who has been off the air for 10 weeks short of a year. Not only that, but the show is no longer on a Saturday. The new Doctor, Peter Davison, debuts in Part 1 of "Castrovalva" on Monday the 4th January, with Part 2 on Tuesday the 5th.
How does new Doctor, new Season compare? Well as far as individual episodes go, this season is watched by a solid minimum of 8 million viewers for the whole of its 13 week run (yes, that's 26 episodes at two a week. The shortest duration of a season of Doctor Who so far).
To put this in perspective, 14 of the 26 episodes are watched by over 9 million viewers, while 5 of them reach over 10 million. There is no particular mid-season dip or peak, and overall Season 19 ends much as it began, just above the 8 million mark. Whether this is due to the new time-slot, the new format of 2 episodes a week, or the new Doctor, is anyone's guess. But overall Season 19 doesn't do badly at all, averaging out at a respectable 9.2 million viewers, beating 3 of Tom Baker's 7 seasons, and all of Jon Pertwee and Patrick Troughton's. It also beats William Hartnell's seasons 1 and 3.