The prophets talk about this coming "day of the Lord," and what they meant by it was that there was coming a time when God was going to act in such a way, it was so remarkable, it was supernatural, it was miraculous, it wasn't just normal, let's say, historical events, but God was going to intervene in history of Israel in such a way that it was called the "day of the Lord." Now, sometimes it would be a miraculous deliverance that would be described as a day of the Lord, and other times it would be some type of judgment that would come that, again, would be the day of the Lord. So, it was some type of remarkable divine intervention in human affairs; this is what they would refer to as the day of the Lord. But all of these things, all of these days of the Lord that would happen, was really pointing to the ultimate day of the Lord, and that was the messianic age. That was the ultimate day of the Lord to the Old Testament prophets. So, even though, like for instance, the destruction of Jerusalem in 586, that was regarded as a day of the Lord, but that was just a type of, again, what was going to happen in the messianic age, and especially the second coming of the Lord. And so, again, the day of the Lord is when God does a special act in human history on behalf of his people, or perhaps even in judgment.