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

Other days

gum

Gum Day
pumpkin

Pumpkin Day
ruby

Ruby Day
kiss a brunette

Kiss A Brunette Day
my little pony

My Little Pony Day
book

Book Day
get funky

Get Funky Day
law

Law Day
pampering

Pampering Day
brow

Brow Day