Christmas

Christmas was a traditional holiday celebrated by many humans on Earth.

History
Although he was an alien, the Doctor developed a fondness and appreciation for the celebration of Christmas. Christmas celebrations existed in as far away a time and place at the planet Puxatornee in the year 3090. (BFA: Flip-Flop) Even the normally serious Sara Kingdom from the year 4000 celebrated the holiday with the First Doctor. (DW: The Daleks' Master Plan)

The Ninth Doctor, jokingly or otherwise, claimed to have left Rose Tyler a red bicycle she had wanted for Christmas many years before (from her perspective) they first met. (DW: The Doctor Dances)

Christmas commemorated the birth of ancient Earth religious leader Jesus Christ. Although the actual date of his birth was not Dec. 25, the Christmas celebration evolved over the millennia from the combination of several other celebrations. Jesus was said to have been born in a barn or shed due to an inn having no room for his family; the Tenth Doctor once claimed it was he who got the last room at the inn. (DW: Voyage of the Damned)

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

The figure of Father Christmas (also known as Santa Claus, Pere Noel etc), a man who delivers presents to young children, became a key part of Christmas. He might (TVC: A Christmas Story) or might not have actually existed.

Earth -- and, specifically the vicinity of London -- experienced a number of major events involving aliens at or around Christmas Day starting in the early 21st century. In 2006, Christmas coincided with the arrival of the Guinevere One space probe at Mars, but this celebration was disrupted by the subsequent 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) (DW: The Christmas Invasion); this event also had personal significance to the Doctor as he experienced it immediately after regenerating into his tenth incarnation. In 2007, the arrival of the Webstar over London panicked the populace. (DW: 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 case of another alien invasion (no invasion occurred, but a spaceship replica of The Titanic nearly crashed into Buckingham Palace). (DW: Voyage of the Damned). Also, the Master's attempt to create the Master Race as well the Time Lords return took place on Christmas as well as the days before it. (DW: The End of Time)

Other incidents at that took place at Christmas have included the Cybermen invasion of Christmas 1851 (DW: The Next Doctor) and an attempt by the Gelth to come through the Cardiff Rift on Christmas Eve 1869 (DW: The Unquiet Dead). Both of these events were thwarted by the Doctor. Apparently some incident occurred in San Francisco at Christmas in the year 2000, but the details are not known. (DW: Doctor Who: The TV Movie)

In many Earth cultures, the "Christmas season" or "Holiday season" usually continued until the turning of the new year, which occurred six days later. Several events of note occurred on December 31, 1999, otherwise known as New Year's Eve, including the Eighth Doctor successfully thwarting an attempt by the Master to steal his remaining regenerations soon after the Doctor had himself regenerated (DW: Doctor Who: The TV Movie); and the staff of Torchwood 3 being murdered, leaving Jack Harkness in charge (TW: Fragments). This latter event occurred when the Torchwood Hub had Christmas decorations on display.

When the human race began to colonise worlds other than Earth, the tradition of Christmas came with them. One example of this is the planet where Sardicktown was located. The first settlers there referred to it as 'The Crystal Feast'. (DW: A Christmas Carol)

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, while 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. (DW: Turn Left)