The Surah takes its name from the very first verse Wan shaqq al-Qamar, thereby implying that it is a Surah in which the word al-Qamar has occurred.
The incident of the shaqq-al-Qamar (splitting of the moon) that has been mentioned in it, determines its period of revelation precisely. The traditionists and commentators are agreed that this incident took place at Mina in Makkah about five years before the Holy Prophet's hijrah to Madinah.
In this Surah the disbelievers of Makkah have been warned for their stubbornness which they had adopted against the invitation of the Holy Prophet (peace and blessings of Allah be upon him). The amazing and wonderful phenomenon of the splitting of the Moon was a manifest sign of the truth that the Resurrection, of which the Holy Prophet was giving them the news, could take place and that it had approached near at hand. The great sphere of the Moon had split into two distinct parts in front of their very eyes. The two parts had separated and receded so much apart from each other that to the on-lookers one part had appeared on one side of the mountain and the other on the other side of it. Then, in an instant the two had rejoined. This was a manifest proof of the truth that the system of the Universe was neither eternal nor immortal, it could be disrupted. Huge stars and Planets could split asunder, disintegrate, collide with each other, and everything that had been depicted in the Quran In connection with the description of the details of Resurrection, could happen. Not only this : it was also a portent that the disintegration of the system of the Universe had begun and the time was near when Resurrection would take place. The Holy Prophet (peace and blessings of Allah be upon him) invited the people's attention to this event only with this object in view and asked them to mark it and be a witness to it. But the disbelievers described it as a magical illusion add persisted in their denial. For this stubbornness they have been reproached in this Surah.
At the outset it has been said: "These people neither believe in the admonition, nor learn a lesson from history, nor affirm faith after witnessing manifest signs with their eyes. Now they would believe only when Resurrection has taken place and they would be rushing out of their graves towards the Summoner on that Day."
Then, the stories of the people of Noah and of 'Ad and Thamud and of the peoples of Lot and the Pharaoh have been related briefly and they have been reminded of the terrible punishments that these nations suffered when they belied and disregarded the warnings given by the Prophets of God. After the narration of each story the refrain that has been provided is : "This Qur'an is an easy means of admonition, which if a nation takes to heart and thereby takes the Right Way, the torment that descended on the former nations could be avoided. But it would indeed be a folly if instead of heeding the admonition through this easy means, one persisted in heedlessness and disbelieved until one was overtaken by the torment itself."
Likewise, after citing admonitory precedents from the history of the former nations, the disbelievers of Makkah have been addressed and warned to this effect: "If you too adopt the same attitude and conduct for which the other nations have already been punished, why will you not be punished for it?Are you in any way a superior people that you should be treated differently from others? Or, have you received a deed of amnesty that you will not be punished for the crime for which others have been punished?And if you feel elated at your great numbers, you will soon see that these very numbers of yours are put to rout (on the battlefield) and on the Day of Resurrection you will be dealt with even more severely."
In the end, the disbelievers have been told that Allah does not need to make lengthy preparations to bring about Resurrection. No sooner does He give a simple command for it than it will take place immediately. Like everything else the Universe and mankind also have a destiny. According to this destiny everything happens at its own appointed time. It cannot be so that whenever somebody gives a challenge, Resurrection is brought about in order to convince him. If you adopt rebellion because you do not see it coming, you will only be adding to your own distress and misfortune. For your record which is being prepared by Divine agents, has not left any misdeed of yours, great or small, unrecorded.
Source: Sayyid Abul Ala Maududi - Tafhim al-Qur'an - The Meaning of the Quran