The oldest festival of Tamil Nadu and South India, Karthigai Deepam has been mentioned and referred to in many ancient works of Tamil literature such as 'Tolkappiyam' that dates back to 2,000 or 2,500 BC, 'Jeevakachintamani', an epic written by Jain poet, Thiruthakka Thevar, in the Sangam period, 'Karnarpadu', 'Kalavazhi Narpadu' dating around 1,000 BC and 'Pazhamozhi'. It falls in the month of Karthigai when the star Krithigai is on the ascendant according to the Tamil Calendar on a full moon day. It is believed that Lord Muruga, the divine light of Lord Shiva, took his form during this month. The ten-days festival is also known as 'the Festival of Lights' and is said to be the extension of the Deepavali festival of India. In some communities, people keep doubling the number of lamps every day from the day of Deepavali till Karthigai Deepam and thus the burning lamps present an enchanting spectacle during the night.

People clean their houses and decorate them with 'Kolam' patterns on the festive occasion. It is only after the 'Deeparathana' (worship ritual) that the lamps are moved to all the parts of the house and lit to create the dreamy look that enchants everyone. During this festival, people also flock to the Annamali Hills (thus it also known as Annamalai Deepam) to worship the Bharani Deepam, the huge lamp that is lit early in the morning on the final day of the festival in the sanctum sanctorum of the shrine. It is said that the flame of the lame does not flicker on this day and reveals the form of Lord Muruga, reaching up to the sky. The Deepam actually a colossal circular metal vessel with a capacity to hold about 2,000 litres of ghee, a height of five and half feet and diameter of five feet. The wick of the lamp itself is made up of 30m of 'Ghada' cloth burnt using 2 kilos of camphor. It is claimed that on the night of 'Karthigai Pournami', when the lamp is lit it can be seen across an area of 35km around the shrine.