An incident at the time of the Khilafat of Umar Al farook (ra), the Sahabah were sitting around and discussing about Laylatul Qadr. Abdullah ibn Masood (ra) was among them but he was remaining silent. Then Umar (ra) asked him, why are you silent? You have knowledge and only reason for you to be silent is that you are young. Then Abdullah ibn Masood (ra) answered, Allah is One and Allah like odd numbers. Allah created 7 Heavens, 7 earths, 7 days, ... (the Imam mentioned some more, i cant recollect)... So I think Laylatul Qadr is on the 7th night of the Last third of Ramadan (ie 27th)