The census, in that part of Jerusalem around that time, were occurring in Spring. Not sure why March was fixed upon but I remember it making sense.
The problem with the bible is that it is such a small part of the story collection surrounding Jesus, etc. There are thousands of 'articles' written during that period by those names in the bible, that are not available to the public... or anyone really.
And they all contradict each other at some point. Even in the bible some stories contradict- apparently Jesus was at both the Wedding at Canae (spelling?) and doing the Loaves and Fishes thing...
The problem is that they are all moral stories, and not necessarily what actually happened.
All the big religious holidays are chosen to coincide with the big festivals of paganism- as it made converting people easier (ie they still got to celebrate at the same times of the year).