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

Other days

boba

Boba Day
gum

Gum Day
eat what you want

Eat What You Want Day
brow

Brow Day
self care

Self Care Day
bounce

Bounce Day
my little pony

My Little Pony Day
healing

Healing Day
tulip

Tulip Day
artists

Artists Day