The apparent meanings of some verses point to the sending down of the entire Quran all at once on the Night of Qadr, which has made this night very important to Muslims.
“The month of Ramadan is the month in which the Quran was sent down…” (Surah Al-Baqarah, verse 185).
We also read in verses 2 and 3 of Surah Ad-Dukhan: I swear by the illustrious Book, that We sent it down during a Blessed Night. We are ever warning.”
There is also the first verse of Surah Al-Qadr: “We sent this (the Holy Quran) down on the Night of Qadr.”
From these verses we can deduce that there is a blessed night named the Night of Qadr in the holy month of Ramadan on which the Holy Quran was sent down.
This is the most blessed night of the year and a time of forgiveness of sins. On this night, the angels descend to earth and the destiny of everyone is determined. “On this Night, the angels and the spirit descend by the permission of their Lord with His decree (to determine everyone’s destiny). Peace it is, till the break of dawn.” (Surah Al-Qadr, verses 4-5)
The Night of Qadr is in the month of Ramadan for sure but the exact day is not known. The nights of the 19th, 21st, 23rd and 27th days of Ramadan have been mentioned as the likely ones, with more emphasis put on the 23rd and 27th. The faithful have been urged to stay awake on these nights and say prayers and recite supplications until dawn. Also, mourning the martyrdom of Imam Ali (AS) is among the rituals of the Night of Qadr. The 21st day of Ramadan is the martyrdom anniversary of the first Imam (AS).
Two Nuzuls of Quran
The Quran has been sent down twice: Once it was sent down in its entirety and then was revealed to the Holy Prophet (PBUH) in a gradual manner during his prophethood. This is confirmed by the apparent meanings of the verses of the Quran as well as many Hadiths.
Allameh Tabatabaei has said about Nuzul of the Quran: The Quran is more sublime than what we comprehend with our usual understanding. It has an integrated truth without having parts and there can be no change in it. Its magnificent status is too sublime for impure thoughts to reach it. The Quran that has been revealed to the Holy Prophet (PBUH) gradually is reliant on that integrated truth and is like an attire for it and an example of it.”
Based on this explanation, the truth of Quran was sent down to the heart of the Holy Prophet (PBUH) all at once on the Night of Qadr and then its words were revealed to the messenger of God gradually during the 23 years of his prophethood.
source:iqna