The Sun reaches its southernmost point on the celestial sphere during the December solstice, which usually occurs around December 21st or 22nd.

This is when the Sun is directly overhead at the Tropic of Capricorn, which is located at 23.5 degrees south latitude.

This is why the December solstice is the shortest day of the year in the Northern Hemisphere and the longest day of the year in the Southern Hemisphere.