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

Other days

bounce

Bounce Day
nae nae

Nae Nae Day
ruby

Ruby Day
coffee

Coffee Day
lash

Lash Day
lisp

Lisp Day
ormille

Ormille Day
lost sock memorial

Lost Sock Memorial Day
first love

First Love Day
stud

Stud Day