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

Other days

brow

Brow Day
boy friend

Boy Friend Day
fiancee

Fiancee Day
youth

Youth Day
polyamory

Polyamory Day
prosecco

Prosecco Day
stress awareness

Stress Awareness Day
ruby

Ruby Day
african immigrant and refugee hiv and hep awareness

African Immigrant And Refugee Hiv And Hep Awareness Day
bow tie

Bow Tie Day