15 August 1963: birth of Con O'Neil, who played April's father in Class.
ii) broadcast anniversary
15 August 1964: broadcast of "Guests of Madame Guillotine", the second episode of the story we now call The Reign of Terror. The Doctor narrowly escapes both the burning barn and a forced labour gang; Ian, Susan and Barbara are imprisoned in the Conciergerie, and, as the episode ends, Susan and Barbara are taken off for the chop...
iii) date specified in canon
15 August 2003: The Seventh Doctor, Ace and Bernice Summerfield visit the Pinehill Crest Hotel in Kent which is hosting three very different events: a cross-stitch convention, an experiment in time travel and... the summoning of the Scourge. (Big Finish audio play, The Shadow of the Scourge, October 2000.)