The guilt filling the whole of Harry's chest like some monstrous, weighty parasite, now writhed and squirmed. Harry could not stand this, he could not stand being himself any more … he had never felt more trapped inside his own head and body, never wished so intensely that he could be somebody; anybody, else …
-- the moment of transformation, the central heart of the saga
After the death of Sirius during the Battle of the Department of Mysteries, and after Voldemort and Dumbledore duel spectacularly in the Atrium, Harry is transported back to Dumbledore’s office by means of a Portkey as dawn breaks. When Dumbledore returns, Harry rages and breaks things while Dumbledore calmly watches. When Harry finally settles down, Dumbledore proceeds to tell him almost everything about the Prophecy, about his connection to Voldemort, and that Harry may have to die in order to defeat the Dark Lord.
Harry returns to Dumbledore's office just before dawn on June 21. In that part of Scotland on that date, the sun rose at 4:27 am.