"Bu ne zaman oldu?"

I agree. In English, if we were asking when a past event occurred, we could use either "When was this?" or "When did this happen?" The difference might be "When was this?" is more common for questions about repeated things. For example: Sally: "We used to take the trolley into town." John: "When was this?"

I don't think they are the same, that's rather "bu ne zamandı?"