The grand Karthigai Deepam festival is celebrated during the Tamil calendar month Karthigai when the Full Moon Day and the star of Karthigai occur on the same day.
This auspicious day has come to be known as Karthigai and celebrated as Thirukkarthigai Deepa Thirunaal.
Lord Siva, the God of destruction as per the Hindu mythology, opened his 3rd eye on his forehead to destroy the demon Soorapadman when sparks were directed to the demon. The sparks got transformed into 6 children on 6 lotus flowers over the Saravana Poigai (a pond) from the Ganges River.
The Karthigai women nourished these 6 children and brought them up. The Goddess Parvathi Devi merged all the 6 children into a single child. Thus, the child had 6 heads and 12 hands on a single body. Lord Parvathi gave the names – Kandhan, Arumugan, Kadamban, and Murugan – to this child.
The Karthigai who brought up the 6 divine children were elevated by Lord Siva as stars in the sky and started the festival of honoring them calling it Karthigai festival to be celebrated with the lighting of lamps and praying them.
When there was a contest between the other 2 Gods Brahma and Vishnu as to who was greater, Lord Siva appeared as a holy fireball between them and told whoever finds the start and of the fireball is the greater.
Then, Lord Vishnu assumed the form of a ‘varaham’ (pig), and Brahma took the form of an annam (swan) and searched for the start and the end of the fireball. Lord Vishnu failed to locate the end and returned. Lord Brahma met the fragrant screw pine flower – Thazhampoo in Tamil. The flower told that it has come from Lord Siva’s head. So, Brahma told with the Thazhampoo’s evidence that he has located Lord Siva’s head.
Because of the bluff by both the Thazhmpoo and Lord Brahma, Lord Siva punished them with 2 curses – that Thazhampoo will not be used henceforth for Lord Siva Pooja and that Lord Brahma will not have any dedicated temple on the Earth.
As per the Hindu mythology, it is observed that the day on which Lord Siva assumed the shape of the fireball is celebrated as Karthigai Deepam.