I don't think Doctor Who has ever had so much going on in such a short space of time in it's history.
Thanks to the TARDIS Eruditorium (a blog discussing the threads of pop-culture narrative through Doctor Who's history, taken chronologically and including spinoffs), I can point to the end of 2006, where Torchwood aired the last episodes of its first season, Doctor Who aired The Runaway Bride Christmas special (with popular comedian guest star Catherine Tate, who was surely there as a one-off
), and Sarah Jane Adventures aired its premiere movie-esque episode Invasion of the Bane, all within a single week.
Perhaps not quite as important in terms of the show's complete history, but certainly worthy of notice that the main program plus its two spinoffs were all airing at roughly the same time, and during the popular (or at least highly visible) block of holiday programming.