Nice discussion going on here ppl! I was also curious about this for quite sometime.. and this is what I found..
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 Murugan.
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 Murugan from one of the six scriptures in His praise, namely: Tirumurukarruppadai, Kandar Kalivenba, Kandar Anubhuti, Kandar Alankaram, Kanda Sashti Kavasam and Skanda Puranam. Giant-sized images of four demons (asuras) are moved on uplifted chariots, and these are symbolically encountered and destroyed by the Lord.
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. This is on the sixth day, i.e., the sashti day. All the other events of the festival take place in the hill temple. On the seventh day, Muruga marries Valli. This divine marriage is celebrated both in the hill temple and in the Periyanayaki Temple.
Hope it helps!