National Day of Prayer Unconstitutional?

So Says a U.S. District Judge.

A federal judge has ruled that the National Day of Prayer -- held by every president since Dwight Eisenhower -- is unconstitutional.
U.S. District Court Judge Barbara Crabb issued a 66-page ruling from her bench in Madison, Wisconsin, that the Day of Prayer violates the First Amendment.
Specifically, she wrote, it violates the so-called establishment clause, which bars the federal government from "establishment of religion." Courts have traditionally interpreted that as the principle of separation of church and state.
The Freedom From Religion Foundation filed litigation against the Bush administration in 2008, challenging the constitutionality of the National Day of Prayer.

Every time one of these rulings comes up I wonder how that squares with Christmas being a National holiday. If the National Day of Prayer is a recognition of a state sponsorship of religion what exactly is making Christmas a national holiday?