nightfall (n.)

c. 1200, "a falling to the ground; a dropping from a height, a descent from a higher to a lower position (as by gravity); a collapsing of a building," from Proto-Germanic *falliz, from the source of fall (v.). Old English noun fealle meant "snare, trap."

Of the coming of night from 1650s. Meaning "downward direction of a surface" is from 1560s, of a value from 1550s. Theological sense, "a succumbing to sin or temptation" (especially of Adam and Eve) is from early 13c.

The sense of "autumn" (now only in U.S. but formerly common in England) is by 1660s, short for fall of the leaf (1540s). Meaning "cascade, waterfall" is from 1570s (often plural, falls, when the descent is in stages; fall of water is attested from mid-15c.). The wrestling sense is from 1550s. Of a city under siege, etc., 1580s. Fall guy is attested by 1906.

late Old English niht (West Saxon neaht, Anglian næht, neht) "the dark part of a day; the night as a unit of time; darkness," also "absence of spiritual illumination, moral darkness, ignorance," from Proto-Germanic *nahts (source also of Old Saxon and Old High German naht, Old Frisian and Dutch nacht, German Nacht, Old Norse natt, Gothic nahts).

The Germanic words are from PIE *nekwt- "night" (source also of Greek nyx "a night," Latin nox, Old Irish nochd, Sanskrit naktam "at night," Lithuanian naktis "night," Old Church Slavonic nosti, Russian noch', Welsh henoid "tonight"), according to Watkins, probably from a verbal root *neg- "to be dark, be night." For spelling with -gh- see fight.  The vowel indicates that the modern English word derives from oblique cases (genitive nihte, dative niht).

The fact that the Aryans have a common name for night, but not for day (q.v.), is due to the fact that they reckoned by nights. [Weekley]

Thus in Old English combinations night was "the night before (a certain day or feast day);" compare German Weihnachten "Christmas," literally "holy night." In early times, the day was held to begin at sunset, so Old English monanniht "Monday night" was the night before Monday, or what we would call Sunday night; so saeterniht "Friday night." The Greeks, by contrast, counted their days by mornings.

To work nights preserves the Old English genitive of time. Night soil "excrement" (1770) is so called because it was removed (from cesspools, etc.) after dark. Night train is attested from 1838; night-school from 1520s; night-life "habitual nocturnal carousing" is attested from 1852.