It's exactly what you think it is.
A brilliant full moon rises at NASA's Kennedy Space Center in Florida in 2017.NASA/Kim Shiflett
Oct. 9 is National Moldy Cheese Day, which may not be the most popular of made-up holidays, not like Groundhog Day or Talk Like a Pirate Day. Still, NASA has found a way to celebrate that doesn't involve eating any moldy cheese.
"In honor of National #MoldyCheeseDay, we want to point out that the Moon is not made of cheese,"
In honor of National #MoldyCheeseDay, we want to point out that the Moon is not made of cheese. Learn what it is actually made of here: https://t.co/0yAyr2w2Vz. But please feel free to celebrate anyway. 🧀— NASA Moon (@NASAMoon) October 9, 2019
The space agency linked to a page that goes into detail on the moon's composition, which involves a metallic core, a mantle and a crust. There's no mention of dairy anywhere here, but NASA still encourages us to celebrate this faux-holiday.
The moon-is-made-of-green-cheese myth may date back hundreds of years, but NASA referred to this joking concept with a masterful April Fools' Day prank in 2002. The agency announced the Hubble Space Telescope had been able to confirm the cheesy moon's expiration date, which was inscribed on the lunar surface. "To be cautious, we should completely devour the Moon by tomorrow," a spokesperson advised.
If you want to celebrate this auspicious day properly, then you should go get yourself some moldy (and preferably stinky) cheese like some Roquefort or a nice Stilton. Then party NASA style with a cheese plate while gazing at the moon.