At night or during the night.
From Old English 'on niht', literally 'on night', composed of the preposition 'on' + 'night'. This archaic formation shows the older pattern of using prepositions where modern English would use different constructions.
Medieval people would say 'anight' where we'd now say 'at night'—this 'a-' prefix appears across Middle English in words like 'aboard', 'afoot', and 'asleep', marking a grammatical pattern that modern English has mostly abandoned.
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