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

Other days

kale

Kale Day
donate your hair

Donate Your Hair Day
slime

Slime Day
pick me

Pick Me Day
tell a fairytale

Tell A Fairytale Day
bow tie

Bow Tie Day
brow

Brow Day
ride the wind

Ride The Wind Day
bounce

Bounce Day
comfy

Comfy Day