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
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)

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