Honestly I didn't even know that was a thing. When I mentioned it to my son, though, he insisted on celebrating by having sushi for lunch.

Kudos to your son for that response. :)

It's a national holiday in Japan (as I learned in connection with this year's edition of the "Festive Tasks" game), originally introduced to celebrate the inauguration of the current Japanese constitution in 1946, and it's apparently a pretty big thing in Japanese schools and academia -- as well as in many cities, with parades, cultural and academic awards ceremonies, etc.

Thanks!