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

Other days

fiancee

Fiancee Day
eat what you want

Eat What You Want Day
cuddle up

Cuddle Up Day
slime

Slime Day
gum

Gum Day
nae nae

Nae Nae Day
pick me

Pick Me Day
nail polish

Nail Polish Day
first love

First Love Day
stud

Stud Day