、。
It got late last night.
Literal
Last-night [topic-は], late was-[explanatory-んです].
昨夜 (さくや) is the formal counterpart to the casual 夕べ — written register, more common in news or formal speech. 遅かった ('was late') as a standalone predicate here is ambiguous: it could mean 'I was late getting home,' 'I was up late,' or 'it was late [as a time of day].' The ~んです gives it an explanatory frame, suggesting the speaker is justifying or clarifying something — 'the reason is, [I got in] late last night.'