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

Other days

fiancee

Fiancee Day
reading

Reading Day
prosecco

Prosecco Day
chocolate milkshake

Chocolate Milkshake Day
martini

Martini Day
unicorn

Unicorn Day
african immigrant and refugee hiv and hep awareness

African Immigrant And Refugee Hiv And Hep Awareness Day
tell ur crush you like them

Tell Ur Crush You Like Them Day
tell a fairytale

Tell A Fairytale Day
hannah

Hannah Day