22 April 1942: birth of Denis Lill, who played Dr. Fendleman in Image of the Fendahl (Fourth Doctor, 1977) and Sir George Hutchinson in The Awakening (Fifth Doctor, 1984).
22 April 1984: birth of Michelle Ryan, who played Christina de Souza in Planet of the Dead (Tenth Doctor, 2009).
22 April 1989: death of Kenny McBain, who directed The Horns of Nimon (Fourth Doctor, 1979-80).
ii) broadcast anniversaries
22 April 1967: broadcast of third episode of The Faceless Ones. The Doctor convinces the Commandant to let him investigate the mysterious disappearances.
22 April 1972: broadcast of third episode of The Mutants. Jo, the Doctor, Cotton and Stubbs are trapped in the caves by the Marshall, with gas closing in.
22 April 2006: broadcast of Tooth and Claw. The Doctor and Rose save Queen Victoria from werewolves; she founds the Torchwood Institute.
(Odd coincidence that Pauline Collins features in two episodes shown exactly 39 years apart.)
22 April 2017: broadcast of Smile. The Doctor takes Bill to see one of Earth's first space colonies, where the inhabitants have supposedly cracked the secret of perpetual happiness. However, they soon discover that the cause of this "happiness" has a very deadly punishment for not following along.
iii) date specified in-universe:
22 April 2011: setting of much of Eleventh Doctor stories The Impossible Astronaut and The Wedding of River Song.