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-universe
25 May 1972: birth of Joseph Serf, the titular Man Who Never Was in the very last story of the Sarah Jane Adventures (2011).