Actually, this one is proclaimed and run by an Australian e-learning company, but it seems to have caught on and the competition raises money for schools (which is good) and UNICEF (meh). You can tell it's not American because they call it "maths" rather than "math."
(Why do the Brits say "maths"? Because the word is a contraction of "mathematics," which is a plural. Thus, technically, the contraction should also be plural. That may appear to make perfect sense, but since we won the Battle of Yorktown and they didn't, we're right.)
Anyway, back to World Math Day. Over 5 million kids from 218 countries are taking part in a 60-second math challenge by logging in to worldmathsday.com and answering a series of addition, subtraction, multiplication, and division problems. That makes it the biggest multiplayer session of the day.