The beginning of the revelation of the Qur'an — the Night of Decree
On a blessed night of Ramadan, Jibril descended upon the Prophet ﷺ in the Cave of Hiraʾ with the first of the Qur'an: "Read in the name of your Lord who created." It was the night that changed the face of history — the Night of Decree, better than a thousand months.
The Prophet ﷺ used to seclude himself in the Cave of Hiraʾ for many nights in worship, until the truth came upon him there: the angel came and said, "Read." He said, "I am not one who reads." The angel pressed him until he could bear no more, then released him: "Read in the name of your Lord who created," and he ﷺ returned with a trembling heart.
The Qur'an itself states the time of the revelation: "The month of Ramadan in which the Qur'an was sent down," "Indeed We sent it down on the Night of Decree." So the beginning of revelation was in Ramadan on the Night of Decree, and the Qur'an continued to descend for twenty-three years according to events.
Allah concealed the exact night that it might be sought in the last ten nights and their odd ones. He ﷺ said: "Seek the Night of Decree in the odd nights of the last ten of Ramadan." Whoever stands it in faith and hope of reward is forgiven his past sins, and it is better than a thousand months.
Note — differing reports on the date: Its being in Ramadan is definitive by the text of the Qur'an; fixing the Night of Decree, however, is concealed. The strongest view is that it moves among the odd nights of the last ten, and 27 is widely held among many without certainty.