When do Summer Solstice and Winter Solstice occur? How do they occur?

The Summer Solstice occurs on June 21. It occurs when the Sun appears directly overhead Tropic of Cancer in the Northern Hemisphere.

The Winter Solstice occurs on December 22. It occurs when the Sun appears directly overhead Tropic of Capricorn in the Southern Hemisphere. The Solstice occurs due to combined effect of revolution and inclination of Earth.