Both, If You're Gutsy
Wear One Sock Inside Out Day is easy. You might even celebrate this one multiple times during the year unwittingly.
Just turn out one sock and put it on. Who's gonna know? That said, there are several ways to get noticed by doing this, should you want to recognition.
For any of this to be valid, you have to be wearing pants or shorts or the equivalent that reveal your ankles easily and often.
Just wear the one sock.
Wear socks that are not of the same pair, especially ones of different height and color.
Wear the inside out sock on your ear.
That idea comes from the old remonstrance: "You must be wearing two pairs of socks - one on your feet and one on your ears."
Never heard that one? Ask your parents if they have.