i) births and deaths
23 February 1918: birth of Bill Strutton, writer of The Web Planet (1965) and the novelisation Doctor Who and the Zarbi.
23 February 1928: birth of Bernard Kay, who played Tyler in The Dalek Invasion of Earth (1964), Saladin in The Crusade (1965), Crossland in The Faceless Ones (1967) and Caldwell in Colony in Space (1971).
23 February 1935: birth of Gerry Davis, script editor of Doctor Who from The Celestial Toymaker (1966) to part 3 of The Evil of the Daleks (1967), co-writer of The Tenth Planet (1966), The Highlanders (1967), and Tomb of the Cybermen (1967-68), and sole writer of Revenge of the Cybermen (1975).
23 February 2009: death of Laurence Payne, who played Johnny Ringo in The Gunfighters (1966), Morix in The Leisure Hive (1980), and Dastari in The Two Doctors (1985).
ii) broadcast anniversaries
23 February 1974: broadcast of first episode of Death to the Daleks. The Tardis lands on Exxilon, suffering a power drain; the Doctor finds a stranded earth ship, Sarah is captured by the natives and the Daleks arrive.
23 February 1982: broadcast of fourth episode of The Visitation. The Doctor and friends pursue the Terileptils to London and destroy their base, leading to a much bigger conflagration.
23 February 1983: broadcast of fourth episode of Terminus; last regular appearance of Sarah Sutton as Nyssa. The Doctor shuts down the engine and Nyssa stays behind to help the Lazars; but the Black Guardian is still angry with Turlough.
23 February 1984: broadcast of first episode of Planet of Fire; first appearance of Nicola Bryant as Peri. Kamelion (remember him?) reappears for the first time in eleven months and brings the Tardis to Lanzarote, where a young woman in a bikini is rescued by Turlough.
23 February 1985: broadcast of "A Fix with Sontarans", a Doctor Who segment of Jim'll Fix It with Colin Baker as the Sixth Doctor, Janet Fielding as Tegan and young Gareth Jenkins saving the earth from, well, Sontarans.
also 23 February 1985: broadcast of second episode of The Two Doctors. The Sixth Doctor, Jamie and Peri pursue the Sontarans and the Second Doctor to Spain, where the anthropophagous and hungry Shockeye captures Peri.
I don't count this as one of the dates when six episodes of Old Who were broadcast, though fans of "A Fix with Sontarans" may differ.