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

Other days

buddy

Buddy Day
ruby

Ruby Day
get funky

Get Funky Day
slime

Slime Day
bounce

Bounce Day
prosecco

Prosecco Day
bow tie

Bow Tie Day
believe

Believe Day
brow

Brow Day
pumpkin

Pumpkin Day