A solstice occurs twice a year, once ~ December 21 and again ~June 21. The solstice is marked as the 2 days when the sun reaches it's extreme highest and extreme lowest points in the noon sky, the longest and the shortest days of the year.
When the northern hemisphere is having it's longest days, in June, the southern hemisphere is having it's shortest days, and visa-versa in December each year. So, our summer solstice in June in the northern hemisphere is winter in the southern hemisphere.
Even though our summer *season* is just starting, as of tomorrow our
days start getting slightly shorter each day until December 21.