solstice day
Technical astronomical term for the day of a solstice (summer or winter). In everyday contexts, 夏至 (げし) or 冬至 (とうじ) are more common.
The solstice day is the day when the sun's declination reaches its maximum or minimum.
From 至 (arrive, extreme) and 日 (sun, day), referring to the day when the sun reaches its extreme declination.