All WDW employees, everyone at all airlines and hotels everywhere, convenience stores / gas stations, not to mention the doctors, police, EMTs, etc. Lots and lots of people work holidays.
I work retail, and although I haven't had to work on Thanksgiving or Christmas yet, I've worked pretty much every Black Friday Christmas Eve, and "Boxing Day." Tradition is just another word for routine. When you've worked dozens of Christmas Eves, work is just another thing you do on that day.
boiler operator's, security guards, bakers, etc, etc. there's tons of them when you think about it.