Laylat al-Qadr is believed to be the night when the Quran was first revealed. All Muslims believe that revelation of the Quran occurred in two phases, with the first phase being the revelation in its entirety on Laylat al-Qadr to the angel Gabriel.
Similarly Lailatul Qadr' is to be found in the last ten odd nights of Ramadan but mostly on the 19th or 21st or 23rd night of Ramadhan. The 19th, according to the Shia belief coincides with the night Ali was attacked in the Mihrab while worshipping in the Great Mosque of Kufa, and died on the 21st of Ramadhan. Shia Muslims worship and regard these three nights as greatly rewarding.
Many Shia Muslims, who make up the largest minority of Islamic followers — including the Ismailis and especially Dawoodi Bohra's — observe Laylat al-Qadr on the night of the 23rd, in keeping with traditions received through Ali and his wife Fatimah, Muhammad's daughter and Fatimid Imams. They further believe that there is no doubt over the date and this is being kept secret by Allah to really testify the follower of Ahl al-Bayt. The tradition is also said to have been articulated by Ja'far al-Sadiq and other Shia Imams.