Kandha Sashti Viratham :
The festival of Kandha Sashti Viratham lasts for six days, starting with the day following the new moon in the month of Aswin, and is extremely important for devotees of Lord Muruga. The ‘sashti’ or sixth day of the full moon and the sixth day of the new moon are favorite days for Lord Muruga. Some devotees of Lord Muruga observe partial or full fast on each Sashti day and throughout the Kanda Sashti period. The Sashti falling in on the waxing moon period in the Hindu lunar month of Aippasi or Aswin (October-November) is the day on which Lord Muruga is believed to have killed a super-demon called Sura Padman. This is commemorated in the six-day festival of Kanda Sashti as a symbol of perpetual destruction of evil by God. Each of these six days is, by custom, set apart to recite the glory of Lord Muruga from one of the six scriptures in His praise, namely: Tirumurukarruppadai, Kandar Kalivenba, Kandar Anubhuti, Kandar Alankaram, Kanda Sashti Kavacam and Kanda Puranam. Giant-sized images of four demons (asuras) are moved on uplifted chariots, and these are symbolically encountered and destroyed by the Lord.
The Surasamharam Battle :
The high point of Kanda Sashti, is on the sixth day when the Surasamharam takes place. At many Murugan temples this is ritually re-enacted, but nowhere is it re-enacted on such a scale as at Tiruchendur, where the actual battle is believed to have taken place in pre-history.
In brief, the Surasamharam goes like this , a huge palanquin bearing the titan Gajamukha ('Elephant-faced') is carried by men of a local caste group down to the beach where he stands and dares Senthil Andavar to come out of His temple and fight. Some say that Gajamukha is "Surapadma's brother". But the most ancient Tamil traditions mention only the terrible and cruel Soor ('Terror' personified) who is described as a shape-shifter who can take any form and who cannot be killed. More recent traditions speak of Surapadma and his three 'brothers' who successively confront Murugan and are annihilated each in turn.
Not one to turn away from a fight, Lord Murugan emerges from His Kanda Sashti Mandapam borne on a smaller palanquin by Brahmin men devotees. In the form of a modest-sized brass likeness of the Lord with His Vel or spear held aloft ready to hurl, He is garlanded with lemons, an essential ingredient of South Indian ritual magic. The asura then runs around his divine Adversary borne by his loudly shouting troops, first clockwise, then counter-clockwise while Lord Senthil Andavar and His troops just remain at ease. Then the asura stands back and suddenly charges face-on but halts and draws back. He does this three times but the Lord is undisturbed. So the asura and his troops haul out and fire a missile (a real firecracker-sized rocket guided by a string between two poles held aloft). The missile heads straight towards the Lord, but something happens and the missile stops and turns back towards its senders, causing a roar of delight from the Lord's supporters. The Lord and His troops then charge at the asura with lances drawn and with full battle cry. One young Iyer among them, who represents Lord Murugan, thrusts his spear at the asura and beheads it. The crowd instantly roars its approval.
Then the asura army withdraws some fifty meters and regroups. A new head (of Simha-mukha, the 'Lion-faced' titan) is mounted on the asura's body and again the same things happen. Four times the procedure is repeated; the fourth time Surapadma himself (or his head rather) is represented by a live cock. When Surapadma is vanquished, symbolically he split by the Lord's Vel into the cock and the peacock, the Lord's banner-symbol and vehicle-totem respectively. On the following day (the seventh) there is the Tiru Kalyanam or marriage of Senthil Andavar to Lord Indra's daughter Devasena or Teyvanai Amman as She is best known - the crowning acknowledgement of the Lord's triumph. This occurs at Tirupparankundram , but the same marriage is also celebrated in grand style at Tiruchendur. For this festival only, the deity of Chinnakumarar comes down from the hill temple, kills the demons (asuras) at the corners of the four direction while on a festive procession around the hill, and triumphantly returns to His abode in the hill. All the other events of the festival take place in the hill temple. This divine marriage is celebrated both in the hill temple and in the Periyanayaki Temple.
source : http://murugan.org/temples/tiruchendur.htm