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

Other days

gum

Gum Day
slime

Slime Day
ask out your crush

Ask Out Your Crush Day
emoji

Emoji Day
cat girl

Cat Girl Day
ruby

Ruby Day
lumpy rug

Lumpy Rug Day
one

One Day
aloxolt

Aloxolt Day
me

Me Day