From the OED:
everyday, n. and a.3. To be met with every day; common, ordinary. Of persons and their attributes: Commonplace, mediocre, inferior.
If the slogan were "Celebrate Every Day," I might be inclined to approve. But it's not. I received a postcard from Elegyrl with this disappointing slogan on it, and more can be found online:
Celebrate Everyday fastpass holder
Alas, Mr. Incredible was right.
Hurray! You're average! Let's go to Disneyland!