Although most people think it occurs only on the 27th of Ramaḍān, Laylatu l-Qadr can occur at any time during the whole year. You must consider every night as possibly being this holy night and every person as possibly being al-Khiḍr (as).
The Qadr Night or Laylat al-Qadr (Arabic: لیلة القدر), variously rendered in English as the Night of Decree, Night of Power, Night of Value, Night of Destiny] or Night of Measures, is, in Islam , the night when the Quran was first sent down from Heaven to the world and also the night when the first verses of the Quran were revealed to the prophet Muhammad. According to many Muslim sources, its exact date is uncertain but it was one of the odd-numbered nights of the last ten days of Ramadan, the ninth month of the Islamic calendar. Since that time, Muslims have regarded the last ten nights of Ramadan as being especially blessed. Muslims believe that the Night of Qadr comes with blessings and mercy of God in abundance, sins are forgiven, supplications are accepted, and that the annual decree is revealed to the angels who carry it out according to God’s Grace.
Shia Muslims similarly believe that Laylat al-Qadr is to be found in the last ten odd nights of Ramadan but mostly on the 19th, 21st or 23rd of Ramadan with 23rd being the most important night. 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. He died on 21 Ramadan. Shia Muslims believe that Ali (the first Shia Imam, and the fourth caliph of the Rashidun Caliphate to Sunnis) had special insight and intimacy with Allah on this night. Imam Sadiq is quoted as saying (Tafsir “al-Burhan”, vol. 4, p. 487):
Once Imam Ali was reciting Surat al-Qadr and his sons, Imam Hasan (a) and Imam Husayn (a) were near him. Imam Husayn (a) asked his father: “Father, how come we feel a different sensation when you recite this surah?” Imam Ali(a) replied, “O son of the Prophet and my son! I know things from this chapter that you are not aware of now. When this surah was sent down to the Prophet he asked me to go to him. When I went to him he recited this surah, then he put his hand on my right shoulder and said: O my brother and my successor! O the leader of my nation after me! O tireless fighter with my enemies! This surah is yours after me, and is for your two sons after you. Gabriel who is my brother among the angels informs me of the events of one year of my nation at the night of Qadr. And after me he will give this information to you. This surah will always have a shining light in your heart and in the heart of your successors until the rising of the dawn of the day of reappearance of Qa’im [the one who rises, a title for the Islamic Messiah, Mahdi].”