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 15th anniversary of Doctor Who took place on 23 November 1978, five days after the broadcast of part four of The Stones of Blood which, incidentally, was also the one-hundredth televised Doctor Who story. To commemorate this, Anthony Read asked David Fisher to write a new scene (expanded by Darrol Blake) featuring Romana and K9 surprising the Fourth Doctor with a birthday cake (and a new identical scarf), celebrating his seven hundred fifty-first birthday. However, producer Graham Williams vetoed this idea as being too self-indulgent and the scene was never shot. Blake had already ordered a cake, and this was eaten by the cast and crew.
 * 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. Commemorative merchandise included the DWMS 25th Anniversary Special.
 * The 10th anniversary of Doctor Who Magazine, beginning in 1979, was commemorated with the release of the DWMS Tenth Anniversary Special in 1989.
 * 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. Commemorative merchandise included the 30th Anniversary Calendar and DWMS Winter 1993.
 * The 35th anniversary of Doctor Who was commemorated with the release of the original Doctor Who website in 1998.
 * The 40th anniversary of Doctor Who was commemorated with the release of Zagreus by Big Finish Productions on 21 November, the broadcast of Doctor Who @40 on 22 and 23 November, the animated serial webcast Scream of the Shalka broadcast from 2 weeks before the anniversary date on 12 November until 3 weeks following the anniversary date on 18 December 2003, and the original series of Doctor Who Unbound.
 * 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. Other productions included An Adventure in Space and Time, The Doctors Revisited and The Ultimate Guide. Commemorative merchandise included the 50th Anniversary Blu-ray Special, the 50th Anniversary Collection, the 50th Anniversary Collector's Edition, The Official 50th Anniversary Annual and Doctor Who - The 50th Anniversary Collection.
 * To commemorate the 50th anniversary of the debut of the Cybermen in 1966's The Tenth Planet, Supremacy of the Cybermen was released by Titan Publishing Group in 2016.
 * Daughter of the Gods was produced by Big Finish to emulate a "5th anniversary of Doctor Who" from 1968. It was released in 2019, as Big Finish marked their own 20th anniversary of producing Doctor Who audios. (BFX: Daughter of the Gods, VOR 128)
 * To commemorate the 50th anniversary of the Master's debut in Terror of the Autons in 1971, Masterful'' was released by Big Finish in 2021.
 * The 40th anniversary of Peter Davison's 1982 debut as the Fifth Doctor was commemorated with the release of Forty by Big Finish in 2022.