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

Other days

lisp

Lisp Day
fiancee

Fiancee Day
artists

Artists Day
cursing

Cursing Day
martini

Martini Day
lumpy rug

Lumpy Rug Day
kiss a ginger

Kiss A Ginger Day
ruby

Ruby Day
believe

Believe Day
ask out your crush

Ask Out Your Crush Day