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

Other days

gum

Gum Day
hoodie

Hoodie Day
my little pony

My Little Pony Day
african immigrant and refugee hiv and hep awareness

African Immigrant And Refugee Hiv And Hep Awareness Day
artists

Artists Day
comfy

Comfy Day
self appreciation

Self Appreciation Day
prosecco

Prosecco Day
believe

Believe Day
nae nae

Nae Nae Day