Friends reunited after thirty years - no one knew what night it was anymore.
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history
A line that travels through the classical canon. Its famous Song-dynasty use is in Su Shi's (苏轼) 《念奴娇·中秋》: 我醉拍手狂歌,举杯邀月,对影成三客……不知今夕何夕. The deeper root is 《诗经·唐风·绸缪》: 今夕何夕,见此良人 - 'what night is this, that I meet such a fine one?' - a marriage-night lyric. In both cases the question marks a moment too charged to place in ordinary time.
usage
今夕 = 'tonight'; 何夕 = 'what night.' Said in moments of rapture, nostalgia, or drunken wonder. 夕 here means 'evening/night,' not 'west.'