I found the answer, at least an authoritative source, for the question I asked about a year ago ::
http://partha-the-prof.blogspot.com/2012/01/32012-when-was-jesus-christ-born.html
You can take a look at ::
http://www.biblicalarchaeology.org/daily/biblical-topics/new-testament/how-december-25-became-christmas/
and learn more about my enigmatic question.
Merry Christmas to all of you !
partha