They don't celebrate only one veteran of course. They're just being sloppy with punctuation.
For the record, Tuesday is Veterans Day, plural. Mr. Rewrite will rely on the U.S. Department of Veterans (plural) Affairs for the style. The Associated Press goes with Veterans Day as well.
Mr. Rewrite will forgive Veterans' Day, plural possessive, which is how he handles Presidents' Day. But Veteran's Day is wrong, wrong, wrong.
That makes it a little disappointing (but also a little fun) to see hundreds of botched references on Google News to Veteran's Day.