Anniversary

An anniversary was a date on which a prior event was commemorated. A type of anniversary was a wedding anniversary. (TV: The Power of Three)

Behind the scenes
The anniversary of Doctor Who, which began with the first episode of An Unearthly Child on 23 November 1963, has been commemorated on several occasions.
 * The 10th anniversary of Doctor Who was commemorated with the broadcast of The Three Doctors, which was broadcast across four episodes from 30 December 1972 to 20 January 1973.
 * The 20th anniversary of Doctor Who was commemorated with the broadcast of The Five Doctors on 23 November 1983.
 * The 25th anniversary of Doctor Who was commemorated with the broadcast of Silver Nemesis, the first episode of which premiered on 23 November 1988.
 * The 30th anniversary of Doctor Who was commemorated with the broadcast of Dimensions in Time, the two parts of which were broadcast in the week following the anniversary date, on 26 and 27 November 1993.
 * The 40th anniversary of Doctor Who was commemorated with the broadcast of Doctor Who @40 on 22 and 23 November 2003.
 * The 50th anniversary of Doctor Who was commemorated with the broadcast of The Day of the Doctor, which was simultaneously screened in cinemas worldwide on 23 November 2013.