`Aishah (May Allah be pleased with her) reported:
The Messenger of Allah (Sallallaahu ‘alaihi wasallam) 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.”
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 do not confirm this view.
Riyad-us-Saliheen – Hadith No : 1192