The Chronology of the Revelation of the Qur'an

The noble Quran is, in fact, the Divine Word.  It is, therefore,  secure in the Preserved Tablet.

 The noble Qur'an says: ( بَلْ هُوَ قُرْآنٌ مَجِيدٌ * فِي لَوْحٍ مَحْفُوظٍ )'(Rather, it is the glorious Qur'an in the reserved Tablet).

Then, from the Preserved Tablet, its descention took place twice.

Once, the whole of it had been sent to al-Bayt al-'lzzah, the most exalted House on the firmament of the world. 

The most exalted House (also known as al-Bayt al-Ma'mur) is a House facing Ka'bah that exists in the firmament as the place of worship for angels. 

This descention took place on the Night of Qadr (rendered as the Night of Power in English). 

The second time it used to be revealed to the Holy Prophet صلي الله علیه وسلم gradually as needed, having reached its completion in twenty three years. 

These two modalities of the Qur'anic revelations become clear through the style of the noble Qur'an itself.

In addition to that, al-Nasa'i, al-Baihai and al-Hakim and others have reported from Sayyidina 'Abdullah ibn 'Abbas رضي الله عنه what can be summed up by saying that the first descention of the noble Qur'an to the firmament of the world took place all at one time and the Holy Prophet صلي الله علیه وسلم was blessed with the second descention gradually. (al Itqan v, I, p. 41)

Explaining the wisdom behind the first descention of the noble Qur'Sn on the firmament facing the world, Imam Abu Shamah has said that it aimed at demonstrating the exalted majesty of the noble Qur'an, and at the same time, it was to tell the angels that this was the last Book of Allah which is ready for descention for the guidance of the people of the earth.

Shaykh al-Zurqani makes yet another point when he says that this two-timed descention also aimed at stressing that this Book is beyond all doubts, and it stands preserved at two more places other than the blessed heart of the Holy Prophet صلي الله علیه وسلم that is, in the Preserved Tablet, and in the Exalted House.

  (Manahil al irfan v 1 p 39).

It is almost agreed by all the scholars that the second gradual descention which was on the heart of the Holy Prophet صلي الله علیه وسلم began when his age was forty years.

 The beginning of this descention, as authentically reported, was in the Night of Qudr, and this was the date on which, some years later, the event of the Battle of Badr came to pass.

 However, nothing definite can be said about the exact date of Ramadan when this Night fell. 

There are some reports which identify that of the seventeenth Ramadan, while others place it on the nineteenth, and still others which indicate the Night of the twentyseventh. (Tafsir Ibn Jarir v. 10, p. 7)

Source: Mariful Quran by Mulana Mufti Mohammad Shafi (English translation).