Which celestial body's position defines the summer solstice?

The Sun’s position defines the summer solstice.

Astronomically, a solstice occurs when the Sun reaches its greatest apparent distance north or south of Earth’s celestial equator. The Northern Hemisphere’s summer solstice happens when the Sun reaches its northernmost declination, near the Tropic of Cancer; in the Southern Hemisphere, the corresponding event occurs in December.

Earth’s axial tilt creates the seasonal pattern. As Earth travels around the Sun, each hemisphere alternately tilts toward the Sun, changing the Sun’s apparent height and the length of daylight. The summer solstice is therefore defined by the Sun’s apparent celestial position, not simply by the day with the earliest sunrise or latest sunset.

Earth is involved because its tilt and orbit create the changing viewpoint, but Earth is not the celestial body whose position defines the event. Venus and the Moon have no role in setting Earth’s summer-solstice date.