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

Other days

promposal

Promposal Day
eat what you want

Eat What You Want Day
boba

Boba Day
self appreciation

Self Appreciation Day
offer

Offer Day
kiss a brunette

Kiss A Brunette Day
martini

Martini Day
ruby

Ruby Day
slime

Slime Day
one

One Day