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

Other days

slime

Slime Day
buddy

Buddy Day
cuddle up

Cuddle Up Day
girl friend

Girl Friend Day
lisp

Lisp Day
boy friend

Boy Friend Day
stud

Stud Day
polyamory

Polyamory Day
hoodie

Hoodie Day
special education

Special Education Day