What is the opposite astronomical event from a hemisphere's summer solstice?

The opposite astronomical event from a hemisphere’s summer solstice is the winter solstice.

A summer solstice occurs when a hemisphere is tilted most toward the Sun. That hemisphere receives its highest midday Sun and longest daylight period of the year. Roughly six months later, Earth’s orientation has carried that hemisphere to its maximum tilt away from the Sun: its winter solstice.

The two events are paired across hemispheres. The June solstice is summer in the Northern Hemisphere and winter in the Southern Hemisphere; the December solstice reverses that arrangement. Thus, “opposite” can mean the seasonal counterpart in the same hemisphere, or the simultaneous event in the other hemisphere.

The solstices are not eclipses, and they are not equinoxes. An equinox occurs when the Sun crosses the celestial equator and day and night are approximately equal. A solstice instead marks the Sun’s northernmost or southernmost apparent position.