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

Other days

bounce

Bounce Day
ask out your crush

Ask Out Your Crush Day
comfy

Comfy Day
pick me

Pick Me Day
nail polish

Nail Polish Day
slime

Slime Day
blanket fort

Blanket Fort Day
lisp

Lisp Day
believe

Believe Day
frozen yogurt

Frozen Yogurt Day