Laylat al-Qadr, the "Night of Power" or "Night of Decree", is the night on which, the Qur'an says, its own revelation began, and which Surah 97 calls "better than a thousand months", a night on which the angels and the Spirit descend and which is peace until dawn. It is understood as the holiest night of the year, on which, in a widespread interpretation, the divine decrees for the year ahead are affirmed, and prayer on it is held to carry immense weight.
Its exact date is deliberately unsettled. The Prophet is reported to have told the community to seek it among the odd nights of the last ten of Ramadan, and the near-universal keeping of the twenty-seventh is a strong convention rather than a certainty; the hiddenness is often explained as an encouragement to devote the whole final ten days to worship. Mosques fill for long night prayer (qiyam) and complete recitations of the Qur'an, and many observe i'tikaf, secluding themselves in the mosque for the last ten days. Because the Islamic year is purely lunar and never intercalated, the night, with the whole of Ramadan, moves back through the solar seasons by about eleven days each year.