The story
The Qurʾān says the night is better than a thousand months. It is the night the first revelation came to Muḥammad in the cave at Hira, when the angel told him to read.
The exact date was deliberately not fixed. The account is that the Prophet was about to name it and an argument among two men made him forget, and he said it might be better that way — because not knowing keeps the whole last ten nights awake.
The word qadr carries both power and measure: what is decreed for the coming year is held to be set down on it.