https://www.cnet.com/news/forget-the-super-bowl-monsters-inc-fans-are-celebrating-2319-day/

As pointed out by CNET sister site ComicBook.com, the scene in question involves the monster George (Sam Black), who gets an unexpected haircut as a result of getting a child's sock stuck on his gloriously orange fur. At this point in the film, monsters think children are toxic, making the sock a big issue code-named 2319. 

Fans who remember the scene have taken to Twitter to celebrate the date, with all sorts of orange-ful posts in memory of George's hair.