25 May 1968: broadcast of fifth episode of The Wheel in Space. The Doctor fights back against the Cybermen, and sends Jamie and Zoe on a dangerous spacewalk.
25 May 1974: broadcast of fourth episode of Planet of the Spiders. Tommy is healed by the crystal, but Sarah and then the Doctor are captured by the spiders.
ii) date specified in canon
25 May 1977: the Sixth Doctor and Frobisher attend the opening of Star Wars (in David McIntee's 1998 novel, Mission: Impractical).