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

Other days

hoodie

Hoodie Day
stud

Stud Day
cursing

Cursing Day
bonang

Bonang Day
blanket fort

Blanket Fort Day
fiancee

Fiancee Day
self care

Self Care Day
boba

Boba Day
cuddle up

Cuddle Up Day
mean girls

Mean Girls Day