Learn Hardcore Croatian: Večeras idemo kući kasno.

Croatian often uses the present tense for near-future plans when there’s a time expression. Večeras (“tonight”) anchors the time, so idemo naturally means “we’re going (tonight).” It’s very common and sounds natural in speech.

Could I say Večeras ćemo ići kući kasno instead?

Yes. Večeras ćemo ići kući kasno (future tense) is correct and slightly more explicit/formal about futurity. In everyday speech, the present (Večeras idemo…) is more common and lighter. Use the future if you want to stress the planning or timeline.

Why is it kući and not kuću or u kuću?

ići kući is the idiomatic way to say “go (home).” Kući functions like an adverb (“homewards”) and doesn’t take a preposition.
ići u kuću means “go into the house” (focus on entering the building).
Bare kuću without a preposition is incorrect here.

What case is kući?

Formally it’s the dative/locative singular of kuća. In this construction it’s best to think of kući as a fixed, adverb-like form meaning “home(wards).”