1986

Events
By this year International Space Command had regular Lunar flights and maintained a tracking station in Antarctica. (DW: The Tenth Planet)

January

 * 28 - The space shuttle Challenger exploded. (NA: Return of the Living Dad, EDA: Father Time)

April

 * 26 - Chernobyl went into meltdown. (DWM: Black Destiny)

December

 * Humans first detected the planet Mondas returning to the solar system, while it apparently was in orbit between Mars and Venus. The manned atmospheric probe Zeus IV was destroyed by the planet's Cyberman inhabitants, and they went on to launch an attack on Earth, starting with the Snowcap base in Antarctica. They planned to use a Z-bomb located there to destroy Earth as one of the two worlds needed to be destroyed to save the other, but when they failed, Mondas was destroyed instead. Present for these events was the First Doctor, and while the Cybermen were defeated, the strain of the conflict led to his subsequent regeneration from his first incarnation into his second. (DW: The Tenth Planet)
 * Twenty years after they helped the First Doctor prevent the return of Mondas in the then-future year of 1986 in their personal timelines, Ben Jackson and Polly Wright - who went their separate ways in life following their return to Earth in 1966 - reunited on the very same night in 1986, which was by then their present. (ST: Mondas Passing)

Unknown dates

 * Toshiko Sato's family moved back to the UK from Japan. (TW: Greeks Bearing Gifts)
 * Pete Tyler and Jackie Tyler married. The event was witnessed secretly by their future daughter Rose Tyler and the Ninth Doctor. (DW: Father's Day)
 * Martha Jones was born. (NSA: Wooden Heart)

January

 * 09 - DWN: Doctor Who - The Gunfighters was first published.

February

 * 02 - Donald Eccles died.
 * 13 - DWN: Doctor Who - The Time Monster was first published.
 * 16 - Howard Da Silva died.

March

 * 13 - DWN: Doctor Who - The Twin Dilemma was first published.

April

 * 10 - DWN: Doctor Who - Galaxy Four was first published.

May

 * 14 - William Lindsay died.
 * 15 - DWN: Doctor Who - Timelash was first published.
 * TC: Turlough and the Earthlink Dilemma was first published. Turlough and the Earthlink Dilemma was the first release of a planned spin-off series of original novels called "The Companions of Doctor Who". As such, this book was the first original novel based upon characters from the TV series (discounting shorter works released in annuals).
 * REF: Doctor Who: The Early Years was first published.
 * 24 - Robert Holmes, widely considered one of Doctor Who's best scriptwriters, died. His last work was Part 13 of DW: The Trial of a Time Lord, aka Part 1 of DW: The Ultimate Foe.
 * Around the time of Holmes' death, Eric Saward resigns as script editor due to a disagreement with John Nathan-Turner over Saward and Holmes' plans for the final Part of the Trial story arc; Nathan-Turner commissions Pip and Jane Baker to write the final chapter.
 * 31 - Sylvia Coleridge died.

June

 * Severn House launched Make Your Own Adventure with Doctor Who, a series of "interactive" books in which readers made decisions every few pages that directed the storyline. The first two books published are Search for the Doctor and Crisis in Space. Later in the year, US publisher Ballantine reissued the books (in different order) under its Find Your Fate line.
 * 11 - Reed De Rouen, who played Pa Clanton in DW: The Gunfighters, died.
 * 12 - DWN: Doctor Who - The Mark of the Rani was first published.
 * 23 - Nigel Stock died.

July

 * 10 - DWN: Doctor Who - The King's Demons was first published.
 * 17 - REF: Travel Without the TARDIS was first published.

August

 * Garden of Evil and Race Against Time, books three and four of the Make Your Own Adventure with Doctor Who line, are first published.
 * 14 - Production concluded on the 14-part DW: The Trial of a Time Lord. Unknown to most at the time, this will also conclude Colin Baker's involvement with the series.
 * 21 - DWN: Doctor Who - Slipback was first published. Adapted from the radio play of the same name, this was the first novelisation of a Doctor Who story not produced for television. (Note: REF: Howe's Transcendental Toybox says 14th August release.)
 * 22 - REF: Doctor Who - Special Effects was first published.
 * Marvel Comics in the United States published the 23rd and final issue of Doctor Who, a monthly series of reprints of comic strips and other material from Doctor Who Magazine. Discounting distribution of imported works from the UK published by Marvel UK and, later, Panini, the next Doctor Who comics published specifically for the American market would not appear until IDW Publishing began its own series of DWM reprints and original works in 2007.

September

 * 06 - DW: The Mysterious Planet Part 1 (aka The Trial of a Time Lord Part 1) was first broadcast, launching the belated Season 23. The series returns to its original 25-minute Part format and the number of Parts per season was increased from 13 to 14. However, given the actual Part length reverted back to 25 minutes, this has resulted in a considerably shortened season in comparison to the 1984 and previous seasons. The Trial of a Time Lord, although not the first season to consist of a single story arc (see The Key to Time), was the first time only one on-screen title would be used for an entire season, although the fanbase and producers would divide the tale into four stories. A new arrangement of the theme music, by Dominic Glynn, was introduced. Lynda Bellingham, as The Inquisitor and Michael Jayston, as The Valeyard, are introduced and become regulars for this season. Sabalom Glitz was introduced. First fall debut for a new season of Doctor Who since 1980.
 * 11 - DWN: Doctor Who - The Savages was first published.
 * TC: Harry Sullivan's War was first published. Harry Sullivan's War was the second of three released under the Companions of Doctor Who spin-off line, and was written by Ian Marter, who also portrayed Harry Sullivan in the TV series more than a decade earlier. Marter died a few weeks later, making this the final book released (but not the final book written) in his lifetime.
 * 13 - DW: The Mysterious Planet Part 2 (aka The Trial of a Time Lord Part 2) was first broadcast.
 * 18 - REF: The Doctor Who File was first published.
 * 20 - DW: The Mysterious Planet Part 3 (aka The Trial of a Time Lord Part 3) was first broadcast.
 * Dennis Spooner died.
 * 27 - DW: The Mysterious Planet Part 4 (aka The Trial of a Time Lord Part 4) was first broadcast.
 * 28 - Denis Carey died.
 * 29 - Doctor Who Collected Comics, a reprinting of several Doctor Who Magazine strips, published by Marvel Comics.

October

 * Mission to Venus and Invasion of the Ormazoids, books five and six of the Make Your Own Adventure with Doctor Who line, are first published. Severn House discontinued the line after these books.
 * According to The Television Companion by Howe and Walker, at some point this month, Colin Baker was informed that his contract to play the Doctor was not being renewed.
 * 04 - DW: Mindwarp Part 1 (aka The Trial of a Time Lord Part 5) was first broadcast.
 * 06 - Around this time, BBC Controller Michael Grade approached the originator of Doctor Who, Sydney Newman, for ideas on how to revvitalise Doctor Who. In a letter dated 6th October, Newman provided some ideas and suggests he be brought on board as executive producer of the series. He also attempts to be credited as the show's creator. Subsequent meetings, however, fail to result in Newman's return to direct involvement in the series.
 * 08 - Michael Obiora was born.
 * 11 - DW: Mindwarp Part 2 (aka The Trial of a Time Lord Part 6) was first broadcast.
 * 16 - DWN: Doctor Who - Fury from the Deep was first published. Promoted as a "bumper volume" due to a higher word count than most Target Books released up to this time.
 * 18 - DW: Mindwarp Part 3 (aka The Trial of a Time Lord Part 7) was first broadcast.
 * 25 - DW: Mindwarp Part 4 (aka The Trial of a Time Lord Part 8) was first broadcast. Nicola Bryant leaves the series with this Part.
 * 28 - Ian Marter, who played companion Harry Sullivan and also wrote numerous novelisations for Target Books, died from a diabetes-related heart attack in London. He leaves behind several Target manuscripts which are published posthumously over the next couple of years.

November

 * REF: Doctor Who: The Companions was first published.
 * 01 - DW: Terror of the Vervoids Part 1 (aka The Trial of a Time Lord Part 9) was first broadcast. Debut of Bonnie Langford as Melanie Bush. Mel became the first companion (aside from his inaugural companion Susan) whose first meeting with the Doctor was never chronicled on television. Honor Blackman, who starred in the Sydney Newman-created The Avengers, guest stars.
 * 08 - DW: Terror of the Vervoids Part 2 (aka The Trial of a Time Lord Part 10) was first broadcast.
 * 11 - Paperback publication of REF: The Doctor Who Cookbook.
 * 15 - DW: Terror of the Vervoids Part 3 (aka The Trial of a Time Lord Part 11) was first broadcast.
 * 20 - DWN: Doctor Who - The Celestial Toymaker was first published.
 * 22 - DW: Terror of the Vervoids Part 4 (aka The Trial of a Time Lord Part 12) was first broadcast.
 * 29 - DW: The Ultimate Foe Part 1 (aka The Trial of a Time Lord Part 13) was first broadcast. This was the final Part to carry a writer credit for the late Robert Holmes, and was also the last Part to credit Eric Saward as script editor.

December

 * US edition of REF: Doctor Who: The Companions was first published.
 * 04 - DWN: Doctor Who - The Seeds of Death was first published.
 * 06 - DW: The Ultimate Foe Part 2 (aka The Trial of a Time Lord Part 14) was first broadcast, concluding Season 23. Although not widely known at the time, this would be Colin Baker's final appearance in the series. This also marked the final use of Dominic Glynn's short-lived theme arrangement and the neon-tubing series logo that had been in place (with adjustments upon Baker's arrival) since 1980. Lynda Bellingham and Michael Jayston, having spent the full season as regular cast members, take their leave.
 * The BBC announced that Colin Baker would not be returning as the Doctor in the next series.
 * 14 - Dermot Tuohy died.
 * 26 - Leslie Dwyer died.