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

Other days

stud

Stud Day
martini

Martini Day
gum

Gum Day
self appreciation

Self Appreciation Day
boyfriend

Boyfriend Day
kale

Kale Day
youth

Youth Day
believe

Believe Day
blanket fort

Blanket Fort Day
donate your hair

Donate Your Hair Day