They will say that Karthigai Deepam belongs to Lord Shiva. But it belongs to Emperuman. This is because the one who provides light to the lamp is God Himself, as stated in the Upanishads. God is the one who gives light to everything, including the sun, moon, and fire. The star Krithika is the star of Agni, and it is appropriate to worship Emperuman, who is the God of Agni, on that day. All these festivals are timeless, because it is the Lord who has been giving them light from time immemorial.