Allah says:

قُلْ يَوْمَ الْفَتْحِ ...

Say: "On the Day of Al-Fath...''

meaning, `when the wrath and punishment of Allah befall you, in this world and the next,'

... لَا يَنفَعُ الَّذِينَ كَفَرُوا إِيمَانُهُمْ وَلَا هُمْ يُنظَرُونَ ﴿٢٩﴾

no profit will it be to those who disbelieve if they (then) believe! Nor will they be granted a respite.

This is like the Ayah,

فَلَمَّا جَآءَتْهُمْ رُسُلُهُم بِالْبَيِّنَـتِ فَرِحُواْ بِمَا عِندَهُمْ مِّنَ الْعِلْمِ...

Then when their Messengers came to them with clear proofs, they were glad with that which they had of the knowledge... (40:83-85)

Those who claim that this refers to the conquest of Makkah go too far, and have made a grievous mistake, for on the day of the conquest of Makkah, the Messenger of Allah accepted the Islam of the freed Makkan prisoners-of-war, who numbered nearly two thousand. If what was meant by this Ayah was the conquest of Makkah, he would not have accepted their Islam, because Allah says:

قُلْ يَوْمَ الْفَتْحِ لَا يَنفَعُ الَّذِينَ كَفَرُوا إِيمَانُهُمْ وَلَا هُمْ يُنظَرُونَ

Say: "On the Day of Al-Fath, no profit will it be to those who disbelieve if they (then) believe! Nor will they be granted a respite.''

What is meant by Al-Fath here is Judgement, as in the Ayat:

فَافْتَحْ بَيْنِى وَبَيْنَهُمْ فَتْحاً

(Nuh said:) So Aftah (judge) between me and them. (26:118)

and:

قُلْ يَجْمَعُ بَيْنَنَا رَبُّنَا ثُمَّ يَفْتَحُ بَيْنَنَا بِالْحَقِّ

Say: "Our Lord will assemble us all together, then He will judge between us with truth.'' (34:26)

وَاسْتَفْتَحُواْ وَخَابَ كُلُّ جَبَّارٍ عَنِيدٍ

And they sought judgement and every obstinate, arrogant dictator was brought to a complete loss and destruction. (14:15)

وَكَانُواْ مِن قَبْلُ يَسْتَفْتِحُونَ عَلَى الَّذِينَ كَفَرُواْ

although aforetime they had invoked Allah to pass judgement over those who disbelieved. (2:89)

إِن تَسْتَفْتِحُواْ فَقَدْ جَآءَكُمُ الْفَتْحُ

If you ask for a judgement, now has the judgement come unto you? (8:19)