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

Other days

chocolate milkshake

Chocolate Milkshake Day
cursing

Cursing Day
ruby

Ruby Day
hoodie

Hoodie Day
first love

First Love Day
get funky

Get Funky Day
boy friend

Boy Friend Day
bonang

Bonang Day
nail polish

Nail Polish Day
boba

Boba Day