Oops, it doesn't look like 'derpy hooves day' is a national day..

Other days

comfy

Comfy Day
law

Law Day
girl friend

Girl Friend Day
lumpy rug

Lumpy Rug Day
boy friend

Boy Friend Day
hot chocolate

Hot Chocolate Day
pumpkin

Pumpkin Day
slime

Slime Day
cuddle up

Cuddle Up Day
gum

Gum Day