Acronychal (sometimes spelled acronical, especially in the US) literally refers to something happening in the evening or at nightfall. However, it’s principally a moderately rare technical term in astronomy and astrology which refers to the rising or setting of a star at the same time as the sun sets.
It’s one of three terms used in reference to star risings and settings; the others are cosmical, of a star which rises or sets at sunrise, and heliacal, of a star which is first seen to rise or set at sunrise or sunset after a period when the event was invisible because it was too close to the sun.
To the ancients, the heliacal rising of the Pleiades — the rising before daybreak — heralded the summer season, while their acronical rising — the rising at sunset — marked the beginning of winter.
The Call of the Stars, by John R. Kippax, 1914.
It derives from the Greek akronikos, nightfall (made up of akros, tip or point, plus nux, night), though it is sometimes misspelled achronychal, as though it had some connection with the Greek khronos, time. The equivalent Latin word is vespertine, which is a relative of vespers, evening prayers.
Very rarely indeed it appears in general writings:
Hard-and-fast rule: When builder R. C. “Chick” Adair tells daytime associates he has acronical obligations, he means he’s a charter member of the Be-Home-by-Dark Club.
The San Antonio Light, 14 Nov. 1967.