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

Other days

lisp

Lisp Day
singles

Singles Day
girl friend

Girl Friend Day
boy friend

Boy Friend Day
lost sock memorial

Lost Sock Memorial Day
cuddle up

Cuddle Up Day
gallery

Gallery Day
fiancee

Fiancee Day
martini

Martini Day
prosecco

Prosecco Day