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

Other days

gum

Gum Day
boba

Boba Day
china

China Day
first love

First Love Day
offer

Offer Day
frozen yogurt

Frozen Yogurt Day
blanket fort

Blanket Fort Day
kiss a brunette

Kiss A Brunette Day
my little pony

My Little Pony Day
believe

Believe Day