"不久前的一个夜晚" can be translated into English as "one night not long ago" or "a night not too long back".
Both expressions convey the meaning of "a night that occurred relatively recently in the past." The first option, "one night not long ago," is more straightforward and commonly used, while the second option, "a night not too long back," has a slightly more casual or conversational tone. Choose the one that best fits the context and tone of your sentence.