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

Other days

pumpkin

Pumpkin Day
artists

Artists Day
bubble bath

Bubble Bath Day
cat girl

Cat Girl Day
blanket fort

Blanket Fort Day
nae nae

Nae Nae Day
tulip

Tulip Day
coffee

Coffee Day
prosecco

Prosecco Day
promposal

Promposal Day