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

Other days

tulip

Tulip Day
bow tie

Bow Tie Day
my little pony

My Little Pony Day
stud

Stud Day
unicorn

Unicorn Day
cursing

Cursing Day
offer

Offer Day
kale

Kale Day
buddy

Buddy Day
bubble bath

Bubble Bath Day