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

Other days

boy friend

Boy Friend Day
prosecco

Prosecco Day
ruby

Ruby Day
law

Law Day
donate your hair

Donate Your Hair Day
one

One Day
brow

Brow Day
ponytail

Ponytail Day
bow tie

Bow Tie Day
slime

Slime Day