Why is Christmas a Federal Holiday?

From the article.

"The Government Didn't "Name it" Christmas
Yes, I see the point that the statute includes the word "Christmas" and that the word has a clear religious connotation (aka "Christ"). But, as the Christmas Wikipedia entry notes, the date of December 25 may have been selected as early as the 4th Century (more than a millennium before the U.S. government existed)! And it was certainly called "Christmas" before the federal statute. 

If the government were to take a random day and call it "Christ Day" then this argument would be more compelling. But the date and name simply didn't originate with the government. Admittedly, the statute does still acknowledge "Christmas" but that is in line with the Court's rationale, which I explain below.

"The Calendar of Public Activities"
If you read the Court's opinion it offers secular purposes for the holiday such as "accommodat[ing] the calendar of public activities" and "recognizing the cultural significance of the holiday." In short, the "public calendar" had a holiday before the government passed the statute, and that holiday was "Christmas." 

full article here.