18 June 1938: birth of Michael Sheard, who played Rhos in The Ark (First Doctor, 1966), Dr. Summers in The Mind of Evil (Third Doctor, 1971), Laurence Scarman in Pyramids of Mars (Fourth Doctor, 1975), Lowe in The Invisible Enemy (Fourth Doctor, 1977), the Mergrave in Castrovalva (Fifth Doctor, 1982), and the Headmaster in Remembrance of the Daleks (Seventh Doctor, 1988). I'm not sure if any other actor has played six different Doctor Who parts of that weight.
18 June 1946: birth of Luan Peters, who played the second version of Chicki in The Macra Terror (Second Doctor, 1967) and Sheila in Frontier in Space (Third Doctor, 1974).
18 June 1973: death of Roger Delgado in a car accident in Turkey; he had played the original Master from 1971-73.
18 June 1991: death of Ronald Allen, who played Rago in The Dominators (Second Doctor, 1968) and Ralph Cornish in The Ambassadors of Death (Third Doctor, 1970).
18 June 2004: death of Frederick Jaeger, who played Jano in The Savages (First Doctor, 1966), Sorenson in Planet of Evil (Fourth Doctor, 1975), and Prof. Marius in The Invisible Enemy (Fourth Doctor, 1977).
ii) broadcast anniversaries
18 June 1966: broadcast of fourth episode of The Savages; last appearance of Peter Purves as Steven. Jano and Chal unite the Elders and the Savages and ask Steven to stay on as their ruler.
18 June 2005: broadcast of The Parting of the Ways, ending the first season of New Who; last appearance of Christopher Eccleston as the Ninth Doctor. The Doctor tries to send Rose back to her own time, but she manages to gain the energy from the Tardis, which she and the Doctor use to destroy the Daleks - at the cost of the Doctor's ninth incarnation.