It is this night in which the first Divine Revelation came to the heart of the Holy Prophet of Islam (PBUH).
Any action done on this night such as reciting the Quran, remembering Allah, etc. is better than acting for one thousand months which do not contain the Night of Qadr.
The exact date of Laylat al-Qadr is unknown. However, according to many traditions, it is in the month of Ramadan. Shias believe that it is whether the eve of 19th or 21st or 23rd of the month of Ramadan. Majority of Shia Muslims put emphasis on the eve of 23rd and majority of Sunni Muslims mark the 27th of the month of Ramadan as Laylat al-Qadr.