Day Zero

Day Zero was the first day of the Year That Never Was, (PROSE: The Story of Martha) the day following the election of, under the name "Harold Saxon", as Prime Minister of the United Kingdom in 2008. (TV: The Sound of Drums)

Preparations
Having announced first contact with the alien Toclafane, in defiance of the United Nations' first contact policy, "Saxon" was confronted by American President Arthur Winters, who insisted that he would make first contact on the following day aboard the UNIT aircraft carrier Valiant. (TV: The Sound of Drums)

Morning
At 8AM, President Winters made first contact with the representatives of the Toclafane, who promptly killed him on the orders of "Harold Saxon". Immediately following this, the Tenth Doctor attempted to step in only to be captured by security, the Master having seen through his perception filter. Also present were the Doctor's companions, Martha Jones and the immortal Jack Harkness, the latter of whom was temporarily killed by the Master's laser screwdriver. The Master then presented Martha's family members, father Clive Jones, mother Francine Jones and sister Tish Jones, whom he had arrested. When the Doctor attempted to reason with him, the Master used his laser screwdriver, with Lazarus Laboratories technology and the biological code derived from the Tenth Doctor's hand, to physically age the Doctor by a hundred years.

At 8:02, the paradox machine within the Doctor's TARDIS went critical, at which point the full force of six billion Toclafane, future descendants of mankind from the year 100,000,000,000,000. (TV: Last of the Time Lords) came down to Earth through a tear in the sky above the Valiant. The Master then gave the order for the Toclafane to remove one tenth of the Earth's population, resulting in millions of humans being slaughtered. Left with Jack Harkness' vortex manipulator, Martha, after being told by the Doctor to "use the countdown", initiated a teleport which took her off the Valiant and down to Earth (TV: The Sound of Drums) in Handcross Park. (PROSE: The Story of Martha)

Reversal
A year later came Launch Day; having overseen the creation of a hundred thousand Toclafane rockets, the Master intended for the Toclafane to begin an invasion of the wider universe, seeking to establish a New Time Lord Empire. Before the ships could be launched, Martha, having allowed herself to be captured by the Master, revealed that she had exploited his own Archangel Network; by travelling the world and telling everyone about the Doctor, she paved the way for the combined thoughts of humanity to rejuvenate the Doctor. Following this, the paradox machine was destroyed by Jack, resulting in the flow of time reversing and negating the past year, the Year That Never Was, leaving the Toclafane at the end of the universe.

Ultimately, time returned to 8:02AM on Day Zero, shortly following the death of President Winters. As the Valiant was the eye of the storm, both it and the people within remained as they were, the only ones aware of the Year That Never Was. Shortly after, the Master was fatally shot by his wife, Lucy Saxon. Despite the Doctor's plea, the Master refused to regenerate and thus died. (TV: Last of the Time Lords)