In some years Diwali is observed a day earlier in Tamil Nadu.
The reason for this is that Diwali in Tamil Nadu is dedicated to Sri Krishna
annihilating Narakasura and it falls on the 14th day of the waning
phase of moon (Krishna Paksha Chaturdashi). Diwali in rest of India is observed on the Amavasya
day or no moon day which falls the next day.
But in some years Chaturdashi Tithi and Amavasya Tithi will
be on a single English day (The auspicious time for both the festival will fall
on a single day as per Gregorian calendar). Then Diwali is observed on the same
day in Tamil Nadu and rest of India .
Diwali in North India is
dedicated to the return of Sri Ram to Ayodhya. In South
India , it is dedicated to Sri Krishna annihilating Narakasura.
Lakshmi Puja which is the most important puja during the
period is observed on the same day in Tamil Nadu and rest of India – on the Amavasya
day.