i) births and deaths
18 April 1928: birth of David Whitaker, the first script editor of Doctor Who (from An Unearthly Child to The Dalek Invasion of Earth) and writer of The Rescue (First Doctor, 1964), The Crusade (First Doctor, 1965), The Power of the Daleks (Second Doctor, 1966), The Evil of the Daleks (Second Doctor, 1966-67), The Enemy of the World (Second Doctor, 1967-68), The Wheel in Space (Second Doctor, 1968) and The Ambassadors of Death (Third Doctor, 1970); also of the 1965 stage play, The Curse of the Daleks, and of two of the first three novelisations, Doctor Who in an Exciting Adventure with the Daleks (1964) and Doctor Who and the Crusaders (1965)
18 April 1929: birth of Peter Jeffreys who played the unnamed Pilot in The Macra Terror (1967) and Count Grendel in The Androids of Tara (1978).
18 April 1930: birth of Angus Lennie, who played the scavenger Storr in The Ice Warriors (Second Doctor, 1967) and the landlord Angus MacRanald in Terror of the Zygons (Fourth Doctor, 1975).
18 April 1956: birth of Eric Roberts, who played the Master in The TV Movie (1996).
18 April 1965: birth of Camille Coduri, who played Rose's mother Jackie Tyler in the first two seasons of New Who.
18 April 1971: birth of David McDonald, better known as David Tennant, who played the Tenth Doctor from 2005 to 2010 and again in 2013, plus Big Finish audios.
18 April 1984: death of Francis De Wolff, who played Vasor in The Keys of Marinus (First Doctor, 1964) and Agamemnon in The Myth Makers (First Doctor, 1965).
ii) broadcast anniversaries
18 April 1964: broadcast of "The Temple of Evil", second episode of the story we now call The Keys of Marinus. Barbara defeats the evil bottled brains.
18 April 1970: broadcast of fifth episode of The Ambassadors of Death. The Doctor blasts off to investigate the Mars probe for himself.
iii) date specified in canon
18 April 1906: the San Francisco earthquake, as featured in Andy Lane's extraordinary 1994 novel, All-consuming Fire.