National Mole Day refers not to the burrowing, worm-eating mammal but rather to a unit of measurement that we were supposed to have learned in Chemistry, all those years ago, namely, the "amount of a substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 (12C), the isotope of carbon with atomic weight 12. This corresponds to a value of 6.02214179(30)×1023 elementary entities of the substance."
And today is 10/23, get it? And National Mole Day really only lasts from 6:02 am to 6:02 pm. Get it, huh, get it?
And every Mole Day has a theme. Last year's theme was "Moles of the Round Table" (you'd think with chemists, the theme might have been "Moles of the Periodic Table," but there are apparently boundaries even chemists won't cross). This year's theme is "Molar Eclipse."
If National Mole Day only has Avogadro's number, not yours, you may wish to consider National TV Talk Show Host Day as an alternative. Determining how this microminiholidayette grew from Johnny Carson's birthday has proved to be beyond the meager skills of our crack research department. You are therefore free to make up any explanation you wish.
Usual Suspect brownielocks.com says that today is also the day that the Swallows Depart From Capistrano. We've heard of the day the swallows return to the Mission at San Juan Capistrano, California (March 19 -- St. Joseph's Day -- in case you're planning ahead), but we were unfamiliar with the idea that there was also a specified departure date. So we learned something.