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

Other days

ruby

Ruby Day
fiancee

Fiancee Day
self care

Self Care Day
blanket fort

Blanket Fort Day
ormille

Ormille Day
comfy

Comfy Day
buddy

Buddy Day
slime

Slime Day
carpenters

Carpenters Day
tell a fairytale

Tell A Fairytale Day