1. The moment when the sun is at its northernmost or southernmost point in the sky then it is the first day of summer or winter.

The summer solstice, when the sun is farthest north, occurs around June 21; the winter solstice takes place around December 21.

2. The point at which the sun reaches its greatest positive and negative declinations.

The referenced dates, summer solstices and winter solstices, are considered the longest and shortest days of the year, respectively.