Many historians now view the Star of Bethlehem as actually being a comet which was observed in 5 BCE.
From the journal of the Royal Astronomical Society:
"The evidence points to Jesus being born in 5 BC March 9 - May 4, probably arond Passover time: 5 BC April 13 - 27. Birth in the Spring is consistent with the account in Luke that there were shepherds livin out in the fields nearby keeping watch over their flocks by night. Birth in 5 BC is also consistent with the census of Caesar Augustus".
http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1991QJRAS..32..389H&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES
So perhaps, if we're to celebrate the birth of Jesus at all (which, as an atheist, I'm not so sure about anyway), we ought to celebrate it bang in the middle of that 13 - 27 April period, i.e. on 20 April. Oh, hang on a moment, that's MY birthday!