18 September 1931: birth of Terence Woodfield, who appeared in two different First Doctor stories in 1966: as Celation in the story we now call The Daleks' Master Plan, and as Maharis in the story we now call The Ark.
18 September 1963: birth of Gary Russell, former editor of Doctor Who Magazine, former producer at Big Finish, author of twelve Doctor Who novels (counting the book-of-the-movie) and of various other related books, script editor for The Waters of Mars and The End of Time, and director of the two Tenth Doctor animated stories.
18 September 2015: Webcast of Incoming Transmission, a prequel for the Youtubed Fan Show starring Dan Starkey as a new incarnation of Co-ordinator Engin.
ii) broadcast anniversaries
18 September 1965: broadcast of "Trap of Steel", the third episode of the series we now call Galaxy 4. The Drahvins hold Steven hostage while the Doctor and Vicki are sent to explore the Rills' ship; and Vicki is horrified when she actually sees one.
18 September 1976: broadcast of third episode of The Masque of Mandragora. Sarah is hypnotised and attempts to stab the Doctor; the Mandragora Helix gathers power.
iii) date specified in-universe
18 September 1945: Albert Einstein is accidentally transported onto the Tardis and briefly turned into an Ood. (Death is the Only Answer, 2011)