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

Other days

gum

Gum Day
kiss a brunette

Kiss A Brunette Day
slime

Slime Day
pumpkin

Pumpkin Day
coffee

Coffee Day
blanket fort

Blanket Fort Day
bow tie

Bow Tie Day
ask out your crush

Ask Out Your Crush Day
singles

Singles Day
bonang

Bonang Day