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

Other days

polyamory

Polyamory Day
aloxolt

Aloxolt Day
china

China Day
camera

Camera Day
prosecco

Prosecco Day
buddy

Buddy Day
self care

Self Care Day
gum

Gum Day
nae nae

Nae Nae Day
unicorn

Unicorn Day