Laylat al-Qadr is celebrated every year in the Muslim world, but the date differs widely. All agree, however, that it occurs in the second half of the month of Ramadan.
Laylat al-Qadr, also known as Shab-e-Qadr, literally means the "Night of Decree" or "Night of Measures", and is the anniversary of two very important dates in Islam that occur during the month of Ramadan.
Muslims believe that it was on this night that God first sent down the first verses of the Qur'an via the Spirit of Revelation Gabriel, to Muhammad when he was forty years old. Other sources say that it is also the anniversary of the night in which the Qur'an was first communicated in its entirety to Muhammad.
The date on which the anniversary falls is a matter of debate. Sunni Muslims generally consider it to be either the 21st, 23rd, 25th, 27th, or 29th of the month of Ramadan, while Shi'as consider it to be the 19th, 21st or 23rd of the month. Shi'as believe the 23rd to be the most important and 21st the least important of the three days.
Conventionally, most Sunnis celebrate it throughout the night of the 27th (i.e. the night between the 26th and 27th). However, due to the uncertainty of the exact date, Muslims are generally recommended to observe all the nights as a matter of precaution.
Mahedvis believe with certainty that its exactly the night of the 27th after the advent of Syed Mohammed Juanpuri, whom some Muslims (also the Zikris) see as the promised Mahdi.
Shia Muslims believe that their fate in the following year is decided on this night, and pray to God all night long, begging for mercy and salvation. This practice is called Ehyaa (the literal meaning of which is "revival").