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)

Original timeline
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 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)

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 Trenzalore with great reluctance. The Doctor and Clara Oswald confronted the Great Intelligence within the Doctor's tomb, 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 history of Trenzalore was changed by the Time Lords. The Eleventh Doctor had grown old and frail after 900 years of fighting the Siege. (PROSE: Tales of Trenzalore: The Eleventh Doctor's Last Stand) With no more plans or lives, he decided to surrender himself to the Daleks, the last enemy remaining at Trenzalore. Clara appealed to the Time Lords for help, stating that "the Doctor" was the only name of his 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. The Doctor used his regeneration energy to destroy the Daleks and save Christmas. Departing Trenzalore in his TARDIS, he regenerated into his twelfth incarnation. (TV: The Time 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)

 Trenzalore