Christmas

Christmas was a traditional holiday celebrated by humans, typically Christians, around the Earth (TV: The End of Time) on 25 December. (TV: Voyage of the Damned) According to Rassilon, it was a pagan rite to banish the cold and the dark. (TV: The End of Time)

People often got together on Christmas, which Danny (in Clara's crab-induced dream) said was because every time they did, it might be the last time. (TV: Last Christmas)

Origin
Christmas commemorated the birth of Earth religious leader Jesus Christ. (TV: Voyage of the Damned) 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 claimed to have left Rose Tyler a red bicycle for Christmas many years before they first met in her personal timeline. (TV: The Doctor Dances, PROSE: The Red Bicycle)

Oliver Cromwell banned Christmas in Britain in the mid-17th century. It was later brought back. (PROSE: The Roundheads, The Feast) 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 the 2000s, the Sto 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 in the 44th century, the first settlers there referred to it as "The Crystal Feast". (TV: A Christmas Carol) Colonists of the planet Mendorax Dellora still celebrated Christmas in the year 5343. (TV: The Husbands of River Song)

A town inhabited 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 aided the Twelfth Doctor and Clara to fight the dream crabs. (TV: Last Christmas)

The Doctor and Christmas
The Doctor has attempted to enjoy the holiday quite a few times, but has always had to deal with a threat of some kind. (TV: The Christmas Invasion, The Runaway Bride, Voyage of the Damned, The Next Doctor, A Christmas Carol, The Doctor, the Widow and the Wardrobe, The Snowmen, Last Christmas) The Tenth Doctor once admitted "All my Christmases are like this." (TV: Voyage of the Damned)

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)

In 1998, the time-active children's toy Parablox was released during the Christmas shopping season. The Great Houses excised it from the timeline shortly after Christmas. (PROSE: The Book of the War)

Parallel world
In a parallel world, 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. (TV: Turn Left)