It is night.
Il est nuit.
Il fait nuit.
or something else?
You can say :
The other two aren't correct.
The correct translation of "It's night." is "Il fait nuit." When you speak about the weather or the sky, you always say "Il fait".
Il fait nuit. (It's night.)
Il fait jour. (It's day.)
Il fait beau. (The weather is good.)
Il fait moche. (The weather is bad.)
Il fait froid. (It's cold.)
Il fait chaud. (It's hot.)
Il fait sombre (The weather is dark. )
Il fait clair. (The weather is bright. )