Diwali / Naraka chaturdhasi

This post is after a recent ( in fact a few mins back) 'big fight' with one of my colleague cum friend. He is a northi and just returned to work after his Diwali vacation. while we were discussing how/why is this festival celebrated, I told the only legend which I knew that 'Krishna killed Narakasura'. Bugger he started laughing on me saying 'how ignorant i can get'.
I was re-assuring several times that this is the actual reason atleast in the South. He was not ready to believe and along joined my manager as well.
He told a story that there is a faith after Ram killed Ravan (which is what is celebrated as Pooja), the day Ram reaches Ayodhy is celebrated as Diwali. That may probably be true which I'm not really sure of.
So now, as always, just did a google search and found this. what i told makes sense n I'm happy :)