*Surah Introduction:54
The Surah talks about the approaching Day of Judgment. It describes some of the scenes of that Day. It also tells us that Allah's judgment may come here and now. There are references to the flood that came at the time of Prophet Noah, the punishment of the people of 'Ad, Thamud, People of Prophet Lot, the Pharaoh and his people. It ends with the good news for the Believers who will be near their Lord in the gardens of bliss.

Sections:

The Day of Judgment is near. References to Prophet Noah, and to the tribe of 'Ad,
References to the Thamud tribe, Prophet Lot and his people.
Pharaoh and his arrogance. The opponents in Makkah are told whether they think they are better or more powerful than those nations. The wrong doers did suffer the consequences of their evil deeds.
This Surah takes its name from the first Ayat, اقْتَرَبَتِ السَّاعَةُ وَانشَقَّ الْقَمَرُ "The Hour has come near, and the moon has split [in two]." (54:1). Surah al-Qamar is in the 27th Juzz and the word Qamar [moon] appears in the Qur'an 27 times. An interesting observation is that the Moon orbits the Earth every 27 days.
There are 55 Ayat in this Surah.

*Central Theme
Theme 1: The Sunnah [practice] of Allah in dealing with those who belie and deny His Ayat [signs].
In this Surah the disbelievers of Makkah have been warned for their stubbornness which they had adopted against the invitation of the Prophet. The amazing and wonderful phenomenon of the splitting of the Moon was a manifest sign of the truth that the Resurrection, of which the 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 Qur’an 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 Prophet 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 and persisted in their denial. For this stubbornness they have been reproached in this Surah.

*Connection of the name of the Surah and its Ayaat
The Surah revolves around the Signs of Allah and those who belie and deny them from amongst the polytheists. The splitting of the Qamar [moon] is amongst the great signs shown by God to the people of Makkah which they still resulted on them continuing their disbelief.

*Connection of the Surah to the Surah before/after it
The previous Surah an-Najm [The Star] and this Surah al-Qamar [The Moon] both are named after heavenly bodies.
There are 55 Ayat in Surah al-Qamar. Surah ar-Rahman is Surah no. 55.
Surah al-Qamar ends with mentioning that the Believers will be with the King. Surah ar-Rahman begins with who this King is - He is ar-Rahman [the Most Merciful]
فِي مَقْعَدِ صِدْقٍ عِندَ مَلِيكٍ مُّقْتَدِرٍ "In a seat of truth (i.e. Paradise), near the Omnipotent King (Allah, the All-Blessed, the Most High, the Owner of Majesty and Honour) (54:55)
الرَّحْمَـٰنُ "The Most Merciful." (55:1)
Both Surah al-Qamar and ar-Rahman end with the mention of Paradise.