After You Figure Out Its Old Use
Most people, if they ever talk about it at all, usually call this mathematical device "calculus". As in, "Did you take calculus in college?"
The normal response to that question: "Calculus is hard."
The proper use of the term though is The Calculus, with that definite article in front of it.
Why? I don't know. You'd think that something as important and mysterious as the calculus could stand on its own. Then again, maybe it's the importance and mysteriosity that earns it the right to be accompanied by the definite article.
Now, what the calculus is normally used for is another question entirely. You also have to decide whether you're talking about differential calculus or integral calculus. (Why isn't it called "differential the calculus" and integral the calculus"?)
We calculate things all the time, but that's not the same as using the calculus.
That (sorta) brings us (finally) to the way to celebrate Find a New Use for The Calculus Day. Since the calculus is so hard, we need to come up with new and hopefully simpler ways to use it.
For starters, I would suggest using it as a bookend.
That's the best I've got today. What about you?