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

Other days

bonang

Bonang Day
bubble bath

Bubble Bath Day
unicorn

Unicorn Day
reading

Reading Day
boba

Boba Day
nail polish

Nail Polish Day
blanket fort

Blanket Fort Day
ride the wind

Ride The Wind Day
slime

Slime Day
boy friend

Boy Friend Day