Fifth Doctor's regeneration

The Fifth Doctor underwent his regeneration into the Sixth Doctor within the control room of his TARDIS as a result of spectrox toxaemia contracted on Androzani Minor. (TV: The Caves of Androzani) This was the fifth regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

A day to come
In an attempt to humiliate the Fifth Doctor, the Tremas Master once suggested he was getting old and that he should soon regenerate. (TV: The King's Demons)

The Fifth Doctor first inadvertently discovered his regeneration was going to occur sooner than expected when he encountered a version of Peri who considered his successor to be "her" Doctor, despite the Sixth Doctor's attempt to prevent this. The Fifth Doctor was left in the knowledge that his regeneration was going to occur while Peri travelled with him. (PROSE: Five Card Draw)

Regenerative process
Succumbing to the poisoning, the Doctor collapsed on the floor of the TARDIS, not entirely sure if he would regenerate, commenting aloud that the regenerative process "[felt] different this time". Visions of Tegan Jovanka, Vislor Turlough, Kamelion, Nyssa, and Adric urged him to live and insisting that the universe still had need of him swirled around his head, countered by a vision of the Tremas Master telling him to die. (TV: The Caves of Androzani)

By one account, these visions took place in a limbo outside of time, no-time. The Doctor’s decision to regenerate would fragment reality, shattering it into “a thousand pieces, a million choices” out of which the Doctor chose survival. (PROSE: The Caves of Androzani)

By another account, the unique nature of this regeneration was due to the Tremas Master trying to interfere via Kamelion's lingering connection to the TARDIS. The Doctor was saved by Nyssa and her husband, Lasarti, who connected with him in his mindscape and helped break the mental link allowing the Master to block his regeneration. (AUDIO: Winter)

The Doctor rejected the Master's assertion that he must die and so regenerated into his sixth incarnation. (TV The Caves of Androzani)

Aftermath
The Sixth Doctor’s recovery from this regeneration was unusually unstable, (TV: The Twin Dilemma) with it being uncertain during his early travels with Peri Brown if he had actually recovered. (PROSE: The Twin Dilemma, The Two Doctors, Attack of the Cybermen)

Legacy
The Doctor's friends relived this regeneration while going backwards through the Doctor's memories to learn about the Other. (PROSE: Lungbarrow)