Oops, it doesn't look like 'flag on its official day' is a national day..

Other days

kale

Kale Day
cuddle up

Cuddle Up Day
bounce

Bounce Day
believe

Believe Day
tulip

Tulip Day
stress awareness

Stress Awareness Day
bow tie

Bow Tie Day
brow

Brow Day
prosecco

Prosecco Day
healing

Healing Day