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

Other days

kale

Kale Day
china

China Day
eat what you want

Eat What You Want Day
believe

Believe Day
cat girl

Cat Girl Day
artists

Artists Day
buddy

Buddy Day
bow tie

Bow Tie Day
prosecco

Prosecco Day
blanket fort

Blanket Fort Day