Event One

Event One, also known as the Big Bang, was a Big Bang. An explosion, the biggest "in history" according to Nyssa (TV: Castrovalva) and the biggest "of all time" according to the Fifth Doctor, (TV: Terminus) which resulted in the creation of the universe, (TV: The Big Bang, Planet of the Dead) and multiverse. (PROSE: The Quantum Archangel) The Beast called it "the Cataclysm". (TV: The Satan Pit)

it occurred in 13,500,017,903 BC. (PROSE: Time Traveller’s Diary)

Pre-universe
At least one universe existed before the one the Doctor lived in existed. (PROSE: The Pit, Millennial Rites, TV: Secrets of the Stars) The Great Old Ones, (PROSE: The Quantum Archangel) the Beast, and the Disciples of the Light, natives of this pre-universe, survived the Big Bang. (TV: The Satan Pit) The Ancient Lights continued some sort of existence after Event One, but were unable to enter the new universe. (TV: Secrets of the Stars)

The Solitract was a form of energy or conscience that existed during the pre universe. It inadvertently prevented the creation of the universe and was exiled into its own plane of existence during Event One. (TV: It Takes You Away)

Before the universe, two beings of good and evil, possibly related to the Great Old Ones, existed and were caught in the explosion. While it is unclear what happened to the good force, the evil force, which later came to be called Fenric, survived and encountered the Doctor in 3rd century Constantinople and in Maiden's Point in 1943. (TV: The Curse of Fenric)

The Second Doctor tricked the Vist into travelling back in time to before the Time Lords evolved so they could claim ownership of time. Instead, they travelled to a point before the Big Bang, where there was no time for them to move across, and were thus trapped there forever. (AUDIO: The Forbidden Time)

The Eleventh Doctor tricked the Kin into thinking he was taking it to the Dawn of Time. Instead, he took the Kin to the moment just before the Big Bang and left it there, in the Void before Creation, where it could do no harm to the universe. Its last word before the Doctor left was, "Doctor!" (PROSE: Nothing O'Clock)

The Doctor's universe
During the first period, there was an "empty void", (TV: Terminus) then matter exploded outward in the form of hydrogen. (TV: Castrovalva) Event One filled the multiverse with new matter, new energy, and new life. (PROSE: The Quantum Archangel)

The Doctor speculated that the massive ship Terminus had time-jumped at some point, discharged its fuel and instigated the Big Bang before time-jumping a second time back from the distant past. (TV: Terminus, PROSE: The Whoniverse)

Another account had a Time Lord from the High Council stating to the Sixth Doctor that the Vipod Mor, headed back in time to the creation of a galaxy, overshot and travelled back to before the universe, thus causing the Big Bang as a "dense monoblock of matter". (AUDIO: Slipback)

The Leptonic Era followed the Big Bang. (TV: Time and the Rani)

A bug-like species nicknamed bookworms fed on debris at the Big Bang and the collapse of the universe. (COMIC: Hunger from the Ends of Time!)

The Big Bang Two
The universe in which the Doctor's adventures took place was destroyed when the Doctor's TARDIS exploded. In order to negate the damage, the Eleventh Doctor used the combination of the exploding TARDIS and the Pandorica to create Big Bang Two, a second Big Bang that effectively rebooted the universe. (TV: The Big Bang)

References to the Big Bang
During the Thousand Year War, a Time Lord told the Fourth Doctor that the Time Lords had mastered transmat technology when "the universe was less than half its present size". (TV: Genesis of the Daleks)

The Seventh Doctor decided to celebrate the creation of the universe, 13,500,020,012 years after it occurred, in 2109. (PROSE: Transit)

The Fourth Doctor implied that he had watched it happen, or at least that other sentient beings had done so. (TV: Destiny of the Daleks)

The delusional Monarch, believing himself God, wished his ship to travel back to the beginning of the universe and witness himself creating it. (TV: Four to Doomsday)

A Vess weapons factory was set in a pocket dimension sustained with energy drawn from the Big Bang. (AUDIO: The Light at the End)

The Tenth Doctor noted that one could potentially sit out the Big Bang as well as the end of the universe and the "start of the next" within a Void Ship. (TV: Army of Ghosts) He later acknowledged to Lady Christina de Souza that he himself had visited both the creation and the end of the universe. (TV: Planet of the Dead)

Luke Smith observed that the Ancient Lights predated the Big Bang. (TV: Secrets of the Stars)

According to the Eleventh Doctor, the Eye of Time was believed by some to be the left-over core of the Big Bang. (GAME: City of the Daleks)

When asked "what happened", the Twelfth Doctor referred to the Big Bang, dinosaurs and bipeds. (TV: The Girl Who Died)

Other information
The White Guardian was most powerful at Event One, where his counterpart, the Black Guardian, could not interfere. (PROSE: The Well-Mannered War)