Oops, it doesn't look like 'bff to my day' is a national day..

Other days

gum

Gum Day
kiss a brunette

Kiss A Brunette Day
promposal

Promposal Day
artists

Artists Day
tulip

Tulip Day
girl friend

Girl Friend Day
cursing

Cursing Day
nail polish

Nail Polish Day
slime

Slime Day
boy friend

Boy Friend Day