In connection with Tamil New Year today, let me re-post the links of my old articles on why Tamil New Year is celebrated on the first day of Chiththirai.
In English:-
http://jayasreesaranathan.blogspot.in/2012/04/gist-of-my-speech-in-chitthirai.html
http://jayasreesaranathan.blogspot.in/2012/04/how-tamil-new-year-and-pongal-were_20.html
In Tamil:-
http://www.tamilhindu.com/2010/01/tamil-new-year-starts-in-chithirai-1/
http://www.tamilhindu.com/2010/01/tamil-new-year-starts-in-chithirai-2/
http://jayasreesaranathan.blogspot.in/2012/04/detailed-reply-to-varalaru-dot-com.html
http://jayasreesaranathan.blogspot.in/2012/04/blog-post_12.html