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

Other days

offer

Offer Day
pumpkin

Pumpkin Day
first love

First Love Day
eat what you want

Eat What You Want Day
girl friend

Girl Friend Day
frozen yogurt

Frozen Yogurt Day
kale

Kale Day
singles

Singles Day
nae nae

Nae Nae Day
buddy

Buddy Day