Oops, it doesn't look like 'assembly of his day' is a national day..

Other days

boy friend

Boy Friend Day
gum

Gum Day
blanket fort

Blanket Fort Day
self appreciation

Self Appreciation Day
fiancee

Fiancee Day
get funky

Get Funky Day
bonang

Bonang Day
one

One Day
ruby

Ruby Day
slime

Slime Day