"This is night."
Translation:C'est la nuit.
Taking the "la" out is an English thing. The French would leave it in.
When you translate to English you typically ignore "le / la" as "the". In French you would say: "Le chat est noir" = " the cat is black"
But you coud also say ( I think my French is right here) "Le chat, le chien, et le oiseau sont noir" which directly translate to "The cat, the dog and the bird are black."
But in English we'd shorten it to "The cat, dog and bird are black."
Would that not translate as "this is of the night", which has a different meaning? "This is night" implies the "this" you're talking about is the nighttime itself, while "this is of the night" implies you're talking about something (an object or animal) which has some affinity with nighttime. Perhaps Batman. "Batman est une créature de la nuit".
That's just from an English speaker's perspective though; correction from a French speaker welcome.
It's ike "this is nighttime". Same as with seasons in that you can say "it's summertime"/"it's summer", for example.
Now, why would one use "this" (or any demonstrative adjcective)? One situation could be pointing at photos, saying "This is day. This is night." It has its uses.
And, back to the seasons. Say you get the first really hot day of summer & tell a friend "Now this is summer!"
Good luck, buddy. Feel free to write me for anything.