Its virtue:

The night of Al-Qadr is the most virtuous night of the year. Allah says in the Qur’an: “We revealed it on the night of power [that is, Al-Qadr]. What will tell you what the night of power is? It is better than a thousand months.” Any action therein, for example, reciting the Qur’an, making remembrance of Allah, and so on, is better than acting for one thousand months which do not contain the night of Al-Qadr.

It is preferred to seek this night:

It is preferred to seek this night during the last ten nights of Ramadan, as the Prophet, peace and blessings be upon him, strove his best in seeking it during that time. We have already mentioned that the Prophet would stay up during the last ten nights, would wake his wives, and then would remain apart from them to worship.

Which night is Laylat ul Qadr?

Scholars hold different opinions as to the night, which is the night of Al-Qadr. Some are of the opinion that it is the 21st, some say the 23rd, others say the 25th and still others say it is the 29th. Some say that it varies from year to year but it is always among the last ten nights of Ramadan. Most scholars, though, vouch for the 27th.
Ahmad recorded, with a Authentic chain, from Ibn Umar that the Prophet said: “He who likes to seek that night should do so on the 27th. ‘Ubayy ibn Kab said: By Allah, and there is no god but Him, it is during Ramadan-and He swore to that-and by Allah, I know what night it is. It is the night during which the Prophet ordered us to make prayers, the night of the 27th. Its sign is that the sun rises in the morning white and without any rays.” (Related by Muslim, Abu Dawud, Ahmad, and by At-Tirmidhi who called it Authentic.)

Praying and making supplications during the night of Al-Qadr:

Al-Bukhari and Muslim record from Abu Hurairah that the Prophet, peace and blessings be upon him, said: “Whoever prays during the night of Al-Qadr with faith and hoping for its reward will have all of his previous sins forgiven.

As to the supplication during the night of Al-Qadr, `A’ishah said: “I asked the Messenger of Allah: ‘O Messenger of Allah, if I know what night is the night of qadr, what should I say during it?’ He said: ‘Say: O Allah, You are pardoning and You love to pardon, so pardon me.’ ” (Related by Ahmad, Ibn Majah, and by At-Tirmidhi, who called it Authentic.)