21 February 1981: death of Ron Grainer, who composed the Doctor Who theme tune. According to the lore, he was so gobsmacked by Delia Derbyshire's electronic arrangement of the music that he asked her, "Did I really write this?" "Most of it," she replied. Of course he got the on-screen credit and she didn't.
ii) broadcast anniversaries
21 February 1970: broadcast of fourth episode of Doctor Who and the Silurians. The Doctor tries to reason with the Silurians but is captured and attacked.
21 February 1976: broadcast of fourth episode of The Seeds of Doom. Keeler starts to turn into the Krynoid; Sarah saves the Doctor from the crusher; Sir Colin and Amelia investigate.
21 February 1981: broadcast of fourth episode of The Keeper of Traken. The Master captures the Doctor and attempts to use the Source to take over his body, but Adric and Nyssa shut down the Source to prevent him. But! The Master takes over Tremas' body and escapes.