"It is We Who have sent down the Qur’an to thee by stages." LXXVI:23
"(It is) a Qur'an which We have divided (into parts from time to time), in order that thou mightest recite it to men at intervals. We have revealed it by stages." XVIL:106
Therefore, it is now clear that the Holy Qur'an was revealed stage by stage for the use of humankind. Why was it not all revealed at one time?
"32 â€‘ Those who reject Faith say: `Why is not the Qur’an revealed to him all at once?' Such (it is revealed) that We may strengthen thy heart thereby, and We have rehearsed it to thee in slow and well arranged stages, gradually. 33 â€‘ And no question do they bring to thee but We reveal to thee the truth and the best explanation (thereof)." XXV:32 & 33
It should be borne in mind that this gradual revelation over some 23 years relates to the Divine revelation to humankind, through Gabriel (PBUH) and Muhammad (SA), which is thought to be the second stage of revelation. The first stage, in contrast, was all at once from the level of the "Mother Book", (Umm ulâ€‘Kitab), before Allah, to that of the "scribe angels" in the heavens of the world, in one night.  Please consider the following ayah from the Glorious Qur'an:
"We have indeed revealed this (Message) in The Grand Night." XCVVII:1
There is more Divine significance about The Grand Night, not known to humankind as yet:
"And what will make you comprehend what The Grand Night is?" XCVII:2
The date of this Excellent Night is also a mystery. Several nights in the blessed month of Ramadan have been suggested with the 27th being the most widely accepted date. All we know confidently is that the Grand Night is in this blessed month, according to the Glorious Qur'an:
"The month of Ramadan, in which was sent down the Qur’an, a guide for humankind, and Clear Signs of guidance and distinction (between virtue and vice)." II:185.