If it's a Federal law, it sure as hell does.
The Post Office--a governmental agency--is closed on christmas; therefore....
"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."
As the Court notes, the days of the week are named for religious reasons. For example, "Thursday" comes from the Norse god Thor. I certainly don't take the statute's recognition of Thanksgiving as "the fourth Thursday in November" as an establishment of Norse polytheism. It just recognizes that the public calendar marks it as such.