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

Other days

frozen yogurt

Frozen Yogurt Day
youth

Youth Day
my little pony

My Little Pony Day
boba

Boba Day
brow

Brow Day
blanket fort

Blanket Fort Day
bonang

Bonang Day
reading

Reading Day
bow tie

Bow Tie Day
slime

Slime Day