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

Other days

lash

Lash Day
nae nae

Nae Nae Day
slime

Slime Day
boy friend

Boy Friend Day
offer

Offer Day
girl friend

Girl Friend Day
hoodie

Hoodie Day
reading

Reading Day
ponytail

Ponytail Day
first love

First Love Day