Trenzalore

Trenzalore was a Level 2 human colony planet, where the village of Christmas was located. Christmas contained the last remaining crack in space-time, through which the Time Lords broadcast the Question and triggered the Siege of Trenzalore. (TV: The Time of the Doctor) In the original timeline, the Siege ended with the death of the Eleventh Doctor and the transformation of his TARDIS into his tomb. (TV: The Name of the Doctor)

Astronomical data
Trenzalore was a blue planet with a human-breathable atmosphere, rings, and two moons, Soror and Frater. (PROSE: Strangers in the Outland) Daylight in the village of Christmas only lasted a few minutes, and it was frequently snowing. (TV: The Time of the Doctor)

In the original timeline, after the Siege of Trenzalore resulted in the Doctor's death, Trenzalore became a barren wasteland covered with molten cracks, and had lost its rings and moons. (TV: The Name of the Doctor)

Trenzalore was of no strategic value to the Nestene Consciousness as it was almost entirely unpolluted, with no toxins, chemicals, acids, metals or smoke in the air. The Doctor was the only reason it had to breach the planet's technology barrier. (PROSE: Strangers in the Outland)

Geography
Christmas was a human town (TV: The Time of the Doctor) where snow was farmed. (PROSE: Let it Snow) Beyond Christmas, other sites included the Doctor's tomb, (TV: The Name of the Doctor) Outland, Devil's Elbows Canyon, the Goat Path, Lake Lagda, Fafnir's Crest, the Choke, (PROSE: Strangers in the Outland) the Glade of Everdell, and Preacher's Clump. (PROSE: Let it Snow)

Before the Doctor
Trenzalore was settled by humans approximately 150 years before the Siege of Trenzalore, at which time the truth field did not exist. (PROSE: The Dreaming) The early settlements stockpiled boronite, which they used to reach underground hot springs by exploding the bedrock that covered them. The leftover boronite was eventually shelved. (PROSE: Strangers in the Outland)

The early colonists modelled Trenzalore on an agrarian society, seeking to recreate the ambience and atmosphere of an earlier time period. Compromises had to be made and not everything could be brought in line with historical reality. (PROSE: The Whoniverse)

The Siege of Trenzalore
The last remaining crack in the universe was located in Christmas. After they were transported to a pocket universe by the Doctor, the Time Lords transmitted the Question "Doctor who?", and a truth field through the crack. If the Doctor answered with his name, they would know that it was safe to return to the universe. (TV: The Time of the Doctor)

The transmission set off the Siege of Trenzalore, in which the Eleventh Doctor made his last stand against hordes of alien races who sought to prevent the return of the Time Lords. (TV: The Time of the Doctor) The planet became a desolate graveyard and the Doctor died, having exhausted his regenerations. His "body" was laid to rest within his tomb: his dying TARDIS that had become gigantic due to the deterioration of its dimension dampeners. (TV: The Name of the Doctor)

The "fall of the Eleventh" upon the fields of Trenzalore became the subject of a prophecy, related to the Eleventh Doctor by Dorium Maldovar. (TV: The Wedding of River Song)



Original timeline
After the Siege, but earlier in the Eleventh Doctor's personal timeline, the Great Intelligence forced him to go to Trenzalore by kidnapping his friends. Due to the possibility of catastrophic paradox, the TARDIS landed on a future Trenzalore with great reluctance. The Doctor and Clara Oswald confronted the Great Intelligence within the Doctor's tomb, his size leaking TARDIS, where instead of a body they found a complex "scar" in space-time that allowed entry into the Doctor's time stream.

The Great Intelligence entered the scar, sacrificing its life to turn all the Doctor's victories into defeats. Clara too entered the scar, splintering herself into many "echoes" to save the Doctor across multiple lifetimes. After his timeline was restored, the Doctor pulled Clara out of the scar, risking the collapse of his entire time stream to do so. (TV: The Name of the Doctor)

After saving Gallifrey from the Last Great Time War, the Eleventh Doctor revealed to the Tenth Doctor that he had visited his tomb on Trenzalore. His younger self commented he didn't want to go there. (TV: The Day of the Doctor)

New timeline
The Eleventh Doctor ended up unknowingly returning to Trenzalore, where he discovered about the Time Lords' transmission. When he realised what planet he was on and afraid of putting Clara in danger in the inevitable war, he tricked her and returned her to Earth. Unable to reason with the Papal Mainframe Mother Superious Tasha Lem -, who asked him not to call back the Time Lords from the pocket universe and restart the Time War - he summoned the whole populace of Christmas and declared that he was the new sheriff in town who would defend them. At this point, The Doctor began fighting the siege. (TV: The Time of the Doctor)

The Doctor eventually grew old and frail after 900 years of fighting. (PROSE: Tales of Trenzalore) Clara returned but was fooled again by the Doctor, once again returning to Earth. With no more plans or lives, the Doctor decided to surrender himself to the Daleks, the last enemy remaining at Trenzalore. Clara, however, returned once more and appealed to the Time Lords for help, stating that "The Doctor" was the only name that mattered. The Time Lords opened the crack and granted the Doctor a new set of regenerations, sealing themselves off from the universe in the process. (TV: The Time of the Doctor) The Doctor used his regeneration energy to destroy the Dalek fleet (PROSE: Twice Upon a Time) and army, saving Christmas. Departing Trenzalore in his TARDIS, he regenerated into the Twelfth Doctor. (TV: The Time of the Doctor)

Behind the scenes
When asked whether the Doctor's averting of his death erased the events of the preceding season, Steven Moffat wrote a monologue in which the Doctor himself answers: "I can change the future so long as the future hasn't been already established as part of my own past. I can't rescue Amy and Rory because I already know I didn't. But what do I know about Trenzalore? There's a big monument that looks very like my TARDIS. There's a temporal fissure leading to my timeline. Maybe it's my grave. Maybe, one day, that's still where I get buried. Maybe it's something else entirely, and we all got it wrong. Don't know. Don't plan to find out for as long as possible. The main thing is, Clara still jumped into my time stream, and still ended up helping me all through my life. All that is established, unchanged – but there's wiggle room!"