21 November, 1964, marks the 50th anniversary of the broadcast of "World's End", the first instalment of The Dalek Invasion of Earth. An Unearthly Child hovered around the 4 million viewer mark; by the end of The Dalek Invasion of Earth, that number would be at over 10 million.
Whoops, didn't mean 'marks' the anniversary, it meant that was the original air date. But you all knew that.
10 million probably wasn't as big a deal in 1964 as it is now. (BBC2 began in April 1964, and as far as I know BBC1 effectively only had one competitor in the form of ITV.) But in fairness the episode probably marked a watershed in the show's history, as it featured the return of a popular enemy that had been definitively beaten in a previous story. A canny move, and one that may have led to the transformation of Doctor Who from a show into a genre.