21 May

21 May was a date.
Historical Earth accounts named this day in 1941 as the last day of the Blitz, (PROSE: A History of Humankind) although other sources suggested it lasted later into the year. (PROSE: Illegal Alien, Just War)
The date was the subject of a mystery that Sarah Jane Smith, the Third Doctor and Jeremy Fitzoliver had to uncover in Italy in 1975. They kept encountering references to the day on their holiday to Brigadier Lethbridge-Stewart's ancestral home. Eventually, the Doctor discovered that it was the date of the return of Clancy's comet. The return amplified the ability of people to cross over into N-Space. (AUDIO: The Ghosts of N-Space)