Oops, it doesn't look like 'flag upside down on day' is a national day..

Other days

bubble bath

Bubble Bath Day
girl friend

Girl Friend Day
lash

Lash Day
stud

Stud Day
self care

Self Care Day
hoodie

Hoodie Day
tulip

Tulip Day
pick me

Pick Me Day
slime

Slime Day
buddy

Buddy Day