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

Other days

lost sock memorial

Lost Sock Memorial Day
offer

Offer Day
ruby

Ruby Day
bounce

Bounce Day
unicorn

Unicorn Day
buddy

Buddy Day
tulip

Tulip Day
frozen yogurt

Frozen Yogurt Day
bow tie

Bow Tie Day
fiancee

Fiancee Day