15 April 1922: birth of Peter Moffatt, who directed State of Decay (1980), The Visitation (1982), Mawdryn Undead (1983), The Five Doctors (1983), The Twin Dilemma (1984) and The Two Doctors (1985).
ii) broadcast anniversaries
15 April 1967: broadcast of second episode of The Faceless Ones. Polly and Ben have disappeared; Jamie meets Samantha Briggs; the Doctor is knocked out by gas.
15 April 1972: broadcast of second episode of The Mutants. The Marshall forces the Doctor to work with Jaeger, while Jo is a captive of the Solonians.
15 April 2006: broadcast of New Earth. The Doctor and Rose visit a far-future hospital where they encounter some former acquaintances.
iii) dates specified in canon
15 April 1912: the Titanic, struck by an iceberg late the previous evening, sinks in the early hours of the morning in the North Atlantic with over 1500 lives lost, as seen in the 1979 DWM comic strip Follow that TARDIS!, Kate Orman's 1993 novel The Left-Handed Hummingbird and the 2010 Big Finish audio Wreck of the Titan, also referenced in The Invasion of Time (1978), Rose (2005), The End of the World (2005) and Voyage of the Damned (2007).
15 April 1912: birth date of Charlotte Pollard, companion of the Eighth and Sixth Doctors in many Big Finish audios.
15 April 1980: birth of Samantha Jones, companion of the Eighth Doctor in many BBC-published novels.