Oops, it doesn't look like 'says this is the happiest day' is a national day..

Other days

Singles Day

Slime Day

African Immigrant And Refugee Hiv And Hep Awareness Day

Eat What You Want Day

Nail Polish Day

Martini Day

Gum Day

Tell Ur Crush You Like Them Day

Nae Nae Day

My Little Pony Day