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

Other days

coffee

Coffee Day
slime

Slime Day
african immigrant and refugee hiv and hep awareness

African Immigrant And Refugee Hiv And Hep Awareness Day
gum

Gum Day
prescription drug take back

Prescription Drug Take Back Day
eat what you want

Eat What You Want Day
stress awareness

Stress Awareness Day
cat girl

Cat Girl Day
hannah

Hannah Day
my little pony

My Little Pony Day