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

Other days

slime

Slime Day
youth

Youth Day
nae nae

Nae Nae Day
lumpy rug

Lumpy Rug Day
lash

Lash Day
bubble bath

Bubble Bath Day
buddy

Buddy Day
boyfriend

Boyfriend Day
my little pony

My Little Pony Day
hoodie

Hoodie Day