18 August 1925: I know this is a bit marginal, but this was the birth date of Brian Aldiss, one of the greatest ever science fiction writers, who died in 2017 the day after his 92nd birthday. At the age of 85, he had a story in the 2011 Brilliant Book of Doctor Who, which may make him the oldest ever Who writer. I was beyond thrilled to meet him in 2014.
ii) Broadcast anniversaries
iii) date specified in canon
18 August 1951: Sarah Jane Smith's parents die in a car accident after attending the village fete at Foxgrove, as we see in The Temptation of Sarah Jane Smith (SJA, 2008).