Today is the day we celebrate the birth of Christ. If you don't believe in Christ than you have nothing to celebrate this day.
And groups like the ACLU who expect Christmas to be celebrated without any religious context--well, they are idiots. To them I say 'bah humbug'. They won't rest until every trace of Christ is eradicated.
But to those who do rejoice in the birth of Messiah I say a hearty "Merry Christmas".