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

Other days

polyamory

Polyamory Day
stud

Stud Day
nail polish

Nail Polish Day
cuddle up

Cuddle Up Day
frozen yogurt

Frozen Yogurt Day
my little pony

My Little Pony Day
hoodie

Hoodie Day
gallery

Gallery Day
ruby

Ruby Day
comfy

Comfy Day