Christmas

Christmas was a traditional holiday celebrated by many humans on Earth on 25 December. (TV: Voyage of the Damned)

Origin
Christmas commemorated the birth of Earth religious leader Jesus Christ. The Eighth Doctor claimed to be present at the event, labelled as the "first Christmas". Since the actual date was not 25 December, he worked the date out from census records in Rome; it was "round about 0|zero BC, AD", in Bethlehem, Judea. He checked all the inns of the town; all of them were packed out, but one at the far end of the town, with a stable attached. He travelled there with Leonardo da Vinci, who didn't dare to go further. (AUDIO: Relative Dimensions) Nonetheless, the Tenth Doctor once claimed it was he who got the last room at the inn. (TV: Voyage of the Damned)

Tradition
Although he was an alien, the Doctor developed a fondness and appreciation for Christmas. Christmas celebrations existed as far away in time and place as the planet Puxatornee in the year 3090. (AUDIO: Flip-Flop) Even the usually serious Sara Kingdom from the year 4000 celebrated the holiday with the First Doctor when they visited 1965 trying to evade the Daleks. (TV: The Daleks' Master Plan)

The Ninth Doctor, perhaps joking, claimed to have left Rose Tyler a red bicycle for Christmas many years before (from her perspective) they first met. (TV: The Doctor Dances)

Christmas activities in Britain included pantomimes. At a UNIT Christmas party, the Third Doctor, Jo Grant and others put on a production of Aladdin. (PROSE: Timewyrm: Revelation, No Future)

In 2000s, the extraterrestrial guide Copper knew a distorted version of the festivity, according to which human beings worshipped "the great god Santa, a creature with fearsome claws, and his wife Mary, and every Christmas Eve, the people of UK [went] to war with the country of Turkey. They then [ate] the Turkey people for Christmas dinner like savages". (TV: Voyage of the Damned)

When the human race colonised other worlds, Christmas came with them. On one planet, the first settlers there referred to it as "The Crystal Feast". (TV: A Christmas Carol) A town inhabitated by humanoids on the planet Trenzalore was called Christmas. (TV: The Time of the Doctor)

Santa Claus
The figure of Father Christmas (also known as Santa Claus, Pere Noel, Jeff, etc), a man who delivered presents to young children, became a key part of Christmas. He might have actually existed. (TV: A Christmas Carol, COMIC: A Christmas Story)

Related events
Earth — and, specifically the vicinity of London — experienced major events involving aliens at or around Christmas Day in the early 21st century. In 2006, Christmas coincided with the arrival of the Guinevere One space probe at Mars. This celebration was disrupted by the Sycorax invasion. This event was later cited by the Tenth Doctor as the moment the people of Earth were irrevocably introduced to the idea of alien life after years of isolated and covered-up incidents. (TV: The Christmas Invasion) In 2007, the arrival of the Webstar over London panicked the populace. (TV: The Runaway Bride) For Christmas 2008, the populace of London (except a few holdouts such as Queen Elizabeth II and Wilfred Mott) voluntarily evacuated the city in anticipation of another alien invasion. No invasion occurred, but a spaceship replica of the Titanic nearly crashed into Buckingham Palace. (TV: Voyage of the Damned) 's attempt to create the Master Race as well as the Time Lords' return from the Last Great Time War took place around Christmas. (TV: The End of Time)

Other incidents that took place at Christmas have included the Cybermen's attempt to cyber-convert humanity in 1851, which was thwarted by the Tenth Doctor (TV: The Next Doctor), the Great Intelligence's first attempt to conquer the Earth which was stopped by the tears of an entire family crying on Christmas Eve over Clara's death (TV: The Snowmen), and an attempt by the Gelth to come through the Cardiff Rift and occupy the body of every human on Christmas Eve 1869, which Gwyneth prevented. (TV: The Unquiet Dead) The Eighth Doctor told Chang Lee to "take a vacation" and warned him to stay away from San Francisco over Christmas 2000. (TV: Doctor Who)

Alternate timeline
In the alternate timeline experienced by Donna Noble due to the manipulations of the Trickster's Brigade, the Tenth Doctor died during the incident with the Webstar at Christmas 2007. The Titanic replica crashed into London at Christmas 2008, destroying the city and flooding most of southern England with radiation, sparking a chain of events leading to the country becoming a police state. This timeline was subsequently negated. (TV: Turn Left)