Where does the sun remain above the horizon all day (never setting) on the december solstice?

Solstitium is a Latin word from where the term solstice came. This means 'the Sun stands still'.

The reason for this is because on this day, the Sun reaches its southern-most position as seen from the Earth. The Sun seems to stand still at the Tropic of Capricorn and then reverses its direction. It's also common to call it the day the Sun turns around.