* Summer solstice (around June 21st in the Northern Hemisphere): The Sun's rays strike the Tropic of Cancer (23.5° North) at their most direct angle. This marks the longest day of the year in the Northern Hemisphere and the shortest day of the year in the Southern Hemisphere.
* Winter solstice (around December 21st in the Northern Hemisphere): The Sun's rays strike the Tropic of Capricorn (23.5° South) at their most direct angle. This marks the shortest day of the year in the Northern Hemisphere and the longest day of the year in the Southern Hemisphere.
So, the Sun's rays hit their southernmost position during the December solstice and their northernmost position during the June solstice.
