courtesy : http://www.sttemple.com/STT/english/karthigai.asp
More about Karthigai Deepam Festival ...
In tamilnadu,Thiruvannamai- Sri Arunachaleswarar temple is famous for Karthigai deepam.
It is believed that every year during the Tamil month of Karthigai in Kiruthigai Star, exactly at 6.00 p.m. Arthanareeswaramurthi presents himself as Jyothi Swaroopa to his devotees at the time of Karthigai Festival 10th day.
Courtesy : googling
So at 6 o' clock people lit their home with lamps to worship the lord himself in the Jyothi form.
Pic : Deepam in my home