Hop To It
This is not the same type of celebration as Labor Day, where you get the day off of work to celebrate that fact that you do work.
National Work Day is a day to do work.
"But I already work every weekday and sometimes more each week," you say.
Yeah, I know. Today, April 11, is just a day when you get to celebrate that. Celebrating can be as simple as going to work and doing your job. If you want to turn today into something more than that, that's up to you.
A celebration after work hours wouldn't be out of place.
If you want to keep it simple and not overexert yourself, just work. If you're a stay-at-home person, just work there.
In any case, thank you for (coming to work and) working today.