There is no question that the Night of Qadr takes place during the month of Ramaḍān; the reason behind this certainty is that the verses of the Quran all express this understanding. For example, one verse states: “The Quran was revealed during the month of Ramaḍān.” Another verse states: “It was revealed during the night of Qadr.”
Yet there is some question as to which night during the nights Ramaḍān this took place. Many different dates have been discussed in the books of Quranic commentary. For example, all of the following dates have been listed: the 1st night, the 17th, the 19th, the 21st, the 23rd, the 27th, and the 29th. Yet the most accepted of these dates according to traditions is that this night is situated in the last third of the month and it is either the 21st or the 23rd night.
In some traditions, it is mentioned that the Prophet (ṣ) would confine himself to the mosque during the last ten days and he would busy himself with worship. In another tradition from Imam Ṣādiq (‘a), it has been mentioned that: The night of Qadr is the 21st or 23rd night. At this point, the narrator insisted and asked the Imam (‘a): What if I am unable to worship on both nights; which night should I choose? The Imam (‘a) replied: How easy are these two nights for what you seek. In many other traditions from the Ahl al-Bayt, the 23rd night has been emphasized. This is while the traditions of the Ahl al-Sunnah have emphasized the 27th night.
a tradition from Imam Ṣādiq (‘a), it has been narrated that: The designation is
on the 19th, the consolidation is on the 21st, and the
decree (is signed) on the 23rd.
This tradition is very interesting in that it combines all of the other
traditions into a unified understanding of the issue.
Some people are of the opinion that the night of Qadr has been left unknown
amongst the days of the year or the days of the month of Ramaḍān in order that
the people spend their time in worship during all of the possible nights.
 Surah Baqarah, verse 185.
 Surah Qadr, verse 1.
 Nūr al-thaqalayn, vol. 5, p. 625, hadith 58.
 «أَلتَّقْدِیْرُ فِى لَیْلَةِ الْقَدْرِ تِسْعَةُ عَشَرَ، وَ الاِبْرامُ فِى لَیْلَةِ اِحْدى وَ عِشْرِیْنَ وَ الاِمْضاءُ فِى لَیْلَةِ ثَلاثُ وَ عِشْرِیْنَ»
 Nūr al-thaqalayn, vol. 5, p. 626, hadith 62.
 Taken from the text: Tafsīr Nemūneh, Ayatullah Makārim Shīrāzī, Dār al-Kutub al-Islāmīyah, 36th Edition, vol. 27, p. 211-213.