It must be declared as a holiday before the end of 2024. It still counts if it is declared as a holiday after the day actually takes place in 2024, e.g., if the U.S. declares June 9 to be "National Rationalussy Day," and the declaration is passed on July 9, it would still count as Yes.

It also counts if there is something else added to the official name (like "National" in the example) or if it is a variation like "Day of Rationalussy", as long as the name still has "Rationalussy" in it.