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

Other days

singles

Singles Day
gum

Gum Day
kale

Kale Day
donate your hair

Donate Your Hair Day
bounce

Bounce Day
carpenters

Carpenters Day
bow tie

Bow Tie Day
polyamory

Polyamory Day
china

China Day
reading

Reading Day