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

Other days

promposal

Promposal Day
one

One Day
slime

Slime Day
martini

Martini Day
my little pony

My Little Pony Day
lash

Lash Day
pick me

Pick Me Day
artists

Artists Day
donate your hair

Donate Your Hair Day
comfy

Comfy Day