23 May 1964: broadcast of “Temple of Evil”, first episode of the story we now call The Aztecs. The Tardis lands in medieval Mexico, where Barbara is received as the reincarnation of the priest Yetaxa.
23 May 1970: broadcast of third episode of Inferno. The Doctor realises that he has travelled to a parallel Earth. (This is the one with the famous eyepatch scene.)
23 May 1988: release of “Doctorin’ the Tardis” by The Timelords (later the KLF). A true cultural millstone milestone.
23 May 2003: webcast of fourth episode of Shada. The Doctor and Chris discover Skagra’s store of genius minds.
I am amused by how short both you and are able to make your job descriptions while mine went on for paragraphs …