June 21st: the summer solstice.
One of two days in the year when the sun is furthest from the celestial equator, and so the difference in length between night and day is at its greatest: 21st June is the longest day of the year, 21st December the shortest. (But because the duration of the earth’s orbit around the sun is slightly longer than the 365-day calendar, those dates can vary.)
On this winter day the sun appears at its lowest point in the sky, and its noontime elevation appears to”stop” in the sky and stay the same for several days before and after the solstice, and so the word finds its origins. Dating back to the mid-13th century, solstice is from the Latin solstitium — “the point at which the sun seems to stand still” — from sol, “sun”, stitium, “a stoppage”, and sistere, “to stand still”. In very early use, the word came into English as sunstead (or, in late Old English, sunstede).
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