die Nacht
night
The period between sunset and sunrise when it is dark.
In der Nacht schlafen die meisten Menschen.
At night, most people sleep.

night
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night
The period between sunset and sunrise when it is dark.
In der Nacht schlafen die meisten Menschen.
At night, most people sleep.

The verb 'übernachten' (to stay overnight) and the adjective 'nächtlich' (nightly, nocturnal) are derived from 'die Nacht' (night). These words are very useful in everyday life.
Wir haben bei Freunden in Berlin übernachtet.
We stayed overnight with friends in Berlin.
There are two ways to express 'nachts' (at night): the adverb 'nachts' describes a general habit. The prepositional phrase 'in der Nacht' refers to a specific night.
Nachts sind alle Katzen grau.
At night, all cats are gray.
The expression 'über Nacht' (overnight) is often not used literally, but figuratively. It then means 'very suddenly' or 'in an extremely short time'.
Er ist mit seinem ersten Roman über Nacht zum Star geworden.
With his first novel, he became a star overnight.
The format 'Die lange Nacht der ...' (The Long Night of ...) is very popular in Germany. For example, there is the 'Lange Nacht der Museen' (Long Night of Museums) or the 'Lange Nacht der Wissenschaften' (Long Night of Sciences), where institutions stay open until late at night.
Am Samstag gehe ich zur Langen Nacht der Museen.
On Saturday I’m going to the Long Night of Museums.
There are several fixed idioms with 'Nacht' (night). 'Die Nacht zum Tag machen' means to party all night. 'Eine schlaflose Nacht haben' means to be unable to sleep.
Auf der Party haben wir die ganze Nacht getanzt und die Nacht zum Tag gemacht.
At the party, we danced all night and turned night into day.
day
'der Tag' and 'die Nacht' are opposites here. 'der Tag' means the light time when the sun is shining or it is bright outside. 'die Nacht' means the dark time between evening and morning. Both words can also have other meanings, but in this contrast the main point is light and dark. In everyday life, 'Tag' is often used for work, school, and outdoor activities, while 'Nacht' is more associated with sleep, rest, or things that happen late.

In der Nacht hört man auf dieser Straße fast keine Autos.
At night, you hardly hear any cars on this street.
Am Tag ist der Park voll mit Kindern und Spaziergängern.
During the day, the park is full of children and people out walking.