Season 21 begins on the 5th of January 1984, 42 weeks after Season 20 finishes on the 16th of March 1983.
Doctor Who is still being broadcast twice a week, but now on a Thursday and Friday. At the start of the season 7.6 million viewers tune in, presumably to see the return of the Silurians and Sea Devils, but then steadily decline over the next 4 weeks, with the opening 4-parter, "Warriors of the Deep", ending at a million less. Things perk up for "The Awakening", but then we see what appears to be a temporarily repeat of the periodicity seen during Season 20 with Part 2 of "The Awakening" and Parts 2 & 4 of "Frontios". The Friday episode appears to be losing 2 million viewers compared to Thursday, and for an episodic serial this is rather strange, indicating that at least 2 million viewers aren't bothered that they're missing half the story. Perhaps it's simply the fact that there's nothing interesting on Thursday evenings, so they may as well watch Doctor Who?
This effect (if indeed it is an effect) only lasts until the Dalek 2-parter, when the perennial pepperpots give the ratings a much needed (though by no means spectacular) boost, with Part 2 of "Resurrection of the Daleks" providing the season's highest-performing episode at 8 million. The next two stories see a return of the ratings gap between Thursday and Friday (although not as pronounced), lasting halfway into the 5th Doctor's swansong.
Ratings pick up for the last two weeks of "The Caves of Androzani" as almost (but not quite) 8 million viewers tune in to see the last of Peter Davison. Once more we note the gap between a story which fandom has consistently rated at the top of "All Time Great" polls, and the relative indifference of the audience at the time.
In a move not seen since Season 4, the new Doctor's first adventure plays out in the same season as his predecessor, and after an inauspicious 7.6 million viewers tune in to see the new guy, ratings slide inexorably, and Season 21 ends on a sad 6.5 million viewers. Overall, Season 21 just beats Season 20 (by 0.2 million), but this sees it comparable to Troughton's 2nd year - Season 5. It finishes lower than every Pertwee and Tom Baker season (with the exception of the Ratings-hammered Season 18).