17 January 1970: broadcast of third episode of Spearhead from Space. Meg Seeley brings the swarm leader to the Doctor, Liz and the Brigadier; meanwhile General Scobie is confronted by his own double.
17 January 1974: publication by Target Books of Doctor Who and the Auton Invasion by Terrance Dicks and Doctor Who and the Cave Monsters by Malcolm Hulke, based on the 1970 stories Spearhead from Space and Doctor Who and the Silurians, kicking off the Target novelisations which remain the single biggest sequence of Doctor Who books.
17 January 1976: broadcast of third episode of The Brain of Morbius. The Doctor restores the Sisters' flame, and Solon forces Sarah to help him operate on Morbius.
17 January 1981: broadcast of third episode of Warriors' Gate. Romana is rescued by the Tharils and brought to join the Doctor at a feast on the other side of the Gate. K9's condition continues to deteriorate.
17 January 2012: broadcast of second part of K9's Question Time, which as noted yesterday is one of the three episodes here: