Other forms: midnights
Midnight is twelve o'clock at night — the moment one day becomes the next. It takes no article and no preposition beyond at: at midnight, before midnight, after midnight.
The confusion worth clearing up is which day it belongs to. Midnight on Friday is ambiguous in ordinary speech, which is why timetables and contracts write 23:59 or 00:01 instead, and why deadlines are usually set at noon. Note that midnight is 12 a.m. and midday is 12 p.m., which is the reverse of what most people guess. The word is also used attributively for anything happening very late: a midnight feast, a midnight showing, the midnight sun of the far north.
1nounmidnightTwelve o'clock at night.
“The shop closes at midnight.”
synonyms: twelve
2adjectivehappeningHappening at or around twelve at night.
“There is a midnight showing on Fridays.”
3nounmiddleThe middle of the night, spoken of loosely.
“They were still arguing well past midnight.”
Too few real examples of this word to estimate which sense is commonest.
midnight is a time of day.
Old English midniht is simply mid plus night, and the same compound exists across the Germanic languages. It is one of the oldest unchanged compounds in English: the two halves have never needed replacing and the joint has never come apart.
Proto-Germanic
*midjanahts — midnight
Old English
midniht
Middle English
midnight
English, today
midnight
From Middle English midnight, from Old English midniht, from Proto-Germanic midjanahts (“midnight”), equivalent to mid- + night. Cognate with stq Midnoacht (“midnight”), Old High German mittinaht (“midnight”), Danish midnat (“midnight”), Swedish midnatt (“midnight”), Icelandic miðnætti (“midnight”). Compare also stq Middernoacht (“midnight”), Dutch middernacht (“midnight”), German Mitternacht (“midnight”).
Etymology from Wiktionary, CC BY-SA