`Aishah (RA) reported: The Messenger of Allah ﷺ used to observe I`tikaf in the last ten days of Ramadan and say, “Seek Lailat-ul-Qadr (Night of Decree) in the odd nights out of the last ten nights of Ramadan.” [Al-Bukhari].
Commentary: We learn from this Hadith that Lailat-ul-Qadr occurs in any of the five odd nights — 21st, 23rd, 25th, 27th and 29th — of the last ten nights of Ramadan. Its exact date has not been revealed for the reason that people keep themselves awake for prayer for a larger number of nights. Had its date been fixed, people would have kept awake for prayer only on that night. There is a general belief among the scholars that it is fixed, on the 27th night of Ramadan. But this is not correct. Ahadith does not confirm this view.
(Riyad Us-Saliheen, Book of Virtues, Chapter 214, No. 1192)