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

Other days

cat girl

Cat Girl Day
ruby

Ruby Day
gum

Gum Day
cursing

Cursing Day
fiancee

Fiancee Day
tell ur crush you like them

Tell Ur Crush You Like Them Day
brow

Brow Day
boy friend

Boy Friend Day
boba

Boba Day
pick me

Pick Me Day