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

Other days

hoodie

Hoodie Day
singles

Singles Day
china

China Day
stud

Stud Day
lisp

Lisp Day
comfy

Comfy Day
stress awareness

Stress Awareness Day
pick me

Pick Me Day
nae nae

Nae Nae Day
self care

Self Care Day