Note: An essay upon 'In That Day' based upon the book of Zechariah is available here.
Of the one hundred and twelve times that the phrase 'in that day' appears in Scripture forty three of them appear in the prophecy of Isaiah. Thus whilst it may be fair to say that the expression is a little more dominant in Zechariah it is the case that Isaiah has more than twice as much to say about that explicit expression than any other book in the Old or New Testament. We may also reasonably assume that understanding that day is one of the keys to unlocking the meaning of Isaiah as it is a theme upon which he clearly was focused.
The fact that Isaiah referenced 'that day' more than forty times should caution us that no one mention of it is adequate to give us a full appreciation of its' meaning. Notwithstanding the first reference to a subject within a book may reasonably be expected to lay one of the foundations upon which the full definition may be built. Certainly in the context of 'that day' as referenced in Isaiah chapter two we find the subject treated in sufficient depth and length that we may reasonably state that this is a fundamental declaration both of the meaning of 'that day' but also of the message of Isaiah.
I believe one of the vital pieces of information provided by Isaiah 2:11,12 on the subject of 'that day' is that it is the same as the 'day of the Lord'. This latter expression occurs 26 times in scripture, four of them in Isaiah. Zephaniah is the book of which this expression is characteristic; the phrase occurring five times in just three chapters. There are some that attempt to build doctrines based upon a distinction of that day and the day of the Lord. However the clear implication of Isaiah 2 is that they are the same.
The other fact that can be garnered unequivocally from the plain reading of this chapter is the nature of the day.
Isa 2:11 The lofty looks of man shall be humbled, and the haughtiness of men shall be bowed down, and the LORD alone shall be exalted in that day.
Note that both aspects of human pride are going to be decimated. The lofty looks or pride of the eyes are going to be humbled. Pride of station or achievement is often accompanied by external and visible signs of pride; they will go. Sometimes the visible trappings of pride may not be manifest but there is still a form of intellectual arrogance that remains. This too is going to be bowed down. I suspect there is deliberate irony in the fact that the Hebrew translated 'humbled' and 'bowed' down in this verse is identical to the Hebrew used to describe these peoples obeisance to idols in Is 2:9.
However the counterpoint to the humiliation of man is glorious. The Lord alone is going to be exalted or lifted up. Thus the picture is not simply of an extant mountain being squashed; it is rather of a pair of balances which are shifting from one balance point to another. For every inch that one pan drops the other rises by two inches relative to the first. Thus the revealing of our God will be glorious not simply because of His own majesty, as incredible as that will be, but also because His revelation will be accompanied by incontrovertible proof as to the worthlessness of man's own achievements.
Whilst differing as to form and degree most of the commentators agree with each other on the points noted so far. Where they diverge markedly is in respect to when this day will be manifest. There are four principle options: at the time of the Babylonian captivity, at the time of the Roman destruction of Jerusalem, at the end of the church age or at the end of the tribulation. I believe however that three of the four possibilities can be eliminated by careful examination of this foundational chapter.
Firstly in Isa 2:1 we are told that this is a vision concerning Judah and Jerusalem. The reference to Judah is particularly striking. We know that there will be a New Jerusalem which is at least partially related to the church. We also know that attempt to link the church and Israel. However I have never seen any justification at all to link Judah to the Church. I believe that it can be seen from this that this is most certainly not a day that culminates the Church or Gospel era. Whatever the interpretation of this passage it must have a distinctly Jewish.
The most compelling proof that the Babylonian exile was not primarily in view is given by the New Testament. The Lord Himself and then Paul clearly affirm that the Day of Judgment is still future. The Roman destruction of Jerusalem would still have been future for those references. However some of the statements regarding 'that day' made in Isaiah simply do not fit with the Roman attack. We are told that a result would be that all nations would turn to peace and that idolatry would disappear. This is just about the opposite of what happened following the Roman victory.
We are left then with the view of Darby, Scofield and most conservative evangelicals that states that 'that day' is still future. Isaiah is looking forward to a time of great trial and tribulation during which the pride of men will be removed. Following that the Lord will establish a kingdom on Earth with the Jew as His close associates and nations operating in obedience to him.
However, whilst I believe the foregoing has given a reasonable explanation of this passage it has actually not given the conclusion that the Spirit wished us to draw. That is provided by Isaiah himself in the final verse of the chapter.
Isa 2:22 Cease ye from man, whose breath is in his nostrils: for wherein is he to be accounted of? (KJV)
The natural context of Isaiah is the huge socio-political upheaval that was taking place during his ministry. I believe Isaiah is really attempting to fundamentally reset the context. Why worry about the thrashings of the nations around you? It is ultimately God that sets the agenda and God who is going to step in to ensure that the agenda is followed. It is from this perspective they we should be stepping into life and the following chapters of Isaiah.