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

Other days

china

China Day
lisp

Lisp Day
bow tie

Bow Tie Day
bounce

Bounce Day
gum

Gum Day
boba

Boba Day
ride the wind

Ride The Wind Day
cursing

Cursing Day
tulip

Tulip Day
artists

Artists Day