The NT several times refers to the "day" of Christ in the context of an impending judgment. In order to understand how this term is used in the NT, it is necessary to look at how an OT precursor of the phrase is used.
The phrase "the day of the Lord" (TDOL) occurs 26 times in the OT, always in the prophetic literature. Six of these occurrences refer to a "day of the Lord's..." X, but not all of these refer to a future event. We begin, in OT order, with Isaiah:
Is. 2:12 For the day of the LORD of hosts shall be upon every one that is proud and lofty, and upon every one that is lifted up; and he shall be brought low...
The first example of TDOL occurs in the context of an oracle which predicts in the "last" or future days a time when "all nations" will flow to the house of the Lord (2:2) and there will be an end of all war (2:4) and proud men are humbled (2:11-17) or try to hide from the judgment of God (2:19ff). This use clearly indicates a picture of final judgment.
Is. 13:6-9 Howl ye; for the day of the LORD is at hand; it shall come as a destruction from the Almighty. Therefore shall all hands be faint, and every man's heart shall melt: And they shall be afraid: pangs and sorrows shall take hold of them; they shall be in pain as a woman that travaileth: they shall be amazed one at another; their faces shall be as flames. Behold, the day of the LORD cometh, cruel both with wrath and fierce anger, to lay the land desolate: and he shall destroy the sinners thereof out of it.
This time, though, TDOL is a judgment directed exclusively against Babylon. Babylon is directly addressed (13:1, 19) and the Medes are said to be the instrument of Babylon's destruction (13:17). For reference, take note as well of how this singular judgment is described:
Is. 13:10-13 For the stars of heaven and the constellations thereof shall not give their light: the sun shall be darkened in his going forth, and the moon shall not cause her light to shine. And I will punish the world for their evil, and the wicked for their iniquity; and I will cause the arrogancy of the proud to cease, and will lay low the haughtiness of the terrible. I will make a man more precious than fine gold; even a man than the golden wedge of Ophir. Therefore I will shake the heavens, and the earth shall remove out of her place, in the wrath of the LORD of hosts, and in the day of his fierce anger.
It is easy to see a parallel to Jesus' pronouncements upon Jerusalem. More such language is found in Isaiah's last reference:
Is. 34:4, 8-10 And all the host of heaven shall be dissolved, and the heavens shall be rolled together as a scroll: and all their host shall fall down, as the leaf falleth off from the vine, and as a falling fig from the fig tree...For it is the day of the Lord's vengeance, and the year of recompenses for the controversy of Zion. And the streams thereof shall be turned into pitch, and the dust thereof into brimstone, and the land thereof shall become burning pitch. It shall not be quenched night nor day; the smoke thereof shall go up for ever: from generation to generation it shall lie waste; none shall pass through it for ever and ever.
Is. 34 is an oracle of judgment against Edom. Note here that the "day" of the Lord is equated with the "year of recompense," thus indicating that "the day of the Lord" isn't associated with a single 24-hour period. (Stuart notes that the phrase likely originated in the ANE conception of an ideal warrior who could vanquish foes in a day; Hosea-Jonah commentary, 231.)
Jer. 46:10 For this is the day of the Lord GOD of hosts, a day of vengeance, that he may avenge him of his adversaries: and the sword shall devour, and it shall be satiate and made drunk with their blood: for the Lord GOD of hosts hath a sacrifice in the north country by the river Euphrates.
The oracle this time is a warning to Egypt (46:2) and to the Pharaoh of Jeremiah's day. Jeremiah also used this phrase in Lamentations:
Lam. 2:22 Thou hast called as in a solemn day my terrors round about, so that in the day of the LORD'S anger none escaped nor remained: those that I have swaddled and brought up hath mine enemy consumed.
In this case, TDOL is used to refer to a past event of judgment, this time upon Jerusalem.
Next up: Ezekiel --
Ezek. 13:5 Ye have not gone up into the gaps, neither made up the hedge for the house of Israel to stand in the battle in the day of the LORD.
This warning is made to Israel's prophets (13:1) and is an admonition against them for being false prophets (13:6-7). These prophets have predicted peace for Jerusalem (13:16). The reference to TDOL is most likely here to the impending Babylonian attack on Jerusalem.
Ezek 30:3 For the day is near, even the day of the LORD is near, a cloudy day; it shall be the time of the heathen.
The oracle this time is against Egypt, and Babylon will be the mechanism of judgment (30:10).
The next five references are from the book of Joel.
Joel 1:15 Alas for the day! for the day of the LORD is at hand, and as a destruction from the Almighty shall it come.
Joel 2:1 Blow ye the trumpet in Zion, and sound an alarm in my holy mountain: let all the inhabitants of the land tremble: for the day of the LORD cometh, for it is nigh at hand;
Joel 2:11 And the LORD shall utter his voice before his army: for his camp is very great: for he is strong that executeth his word: for the day of the LORD is great and very terrible; and who can abide it?
Joel 2:31 The sun shall be turned into darkness, and the moon into blood, before the great and the terrible day of the LORD come.
Joel 3:14 Multitudes, multitudes in the valley of decision: for the day of the LORD is near in the valley of decision.
Because of the difficulty in knowing when Joel was written, it is not possible to say in all cases who the "enemy" is. It is enough to say that the first three references are to an impending judgment against the Jews.
The next three references to TDOL are from Amos, who was probably the first of the prophets chronologically to use the phrase.
Amos 5:18-20 Woe unto you that desire the day of the LORD! to what end is it for you? the day of the LORD is darkness, and not light. As if a man did flee from a lion, and a bear met him; or went into the house, and leaned his hand on the wall, and a serpent bit him. Shall not the day of the LORD be darkness, and not light? even very dark, and no brightness in it?
Amos 5-6 is a prophetic oracle against the northern kingdom of Israel.
Little Obadiah makes the next use of the phrase:
Obad. 15 For the day of the LORD is near upon all the heathen: as thou hast done, it shall be done unto thee: thy reward shall return upon thine own head.
Obadiah's oracle is upon Edom, and it is clear that Obadiah expects TDOL to affect Edom.
Now Zephaniah takes his turn:
Zeph. 1:8 Hold thy peace at the presence of the Lord GOD: for the day of the LORD is at hand: for the LORD hath prepared a sacrifice, he hath bid his guests. And it shall come to pass in the day of the LORD'S sacrifice, that I will punish the princes, and the king's children, and all such as are clothed with strange apparel.
Zeph. 1:14 The great day of the LORD is near, it is near, and hasteth greatly, even the voice of the day of the LORD: the mighty man shall cry there bitterly.
Zeph. 1:18-2:3 Neither their silver nor their gold shall be able to deliver them in the day of the LORD'S wrath; but the whole land shall be devoured by the fire of his jealousy: for he shall make even a speedy riddance of all them that dwell in the land. Gather yourselves together, yea, gather together, O nation not desired; Before the decree bring forth, before the day pass as the chaff, before the fierce anger of the LORD come upon you, before the day of the LORD'S anger come upon you. Seek ye the LORD, all ye meek of the earth, which have wrought his judgment; seek righteousness, seek meekness: it may be ye shall be hid in the day of the LORD'S anger.
These oracles are against Jerusalem and Judah (1:14). Again it is worth noting the symbolism used:
Zeph. 1:15-17 That day is a day of wrath, a day of trouble and distress, a day of wasteness and desolation, a day of darkness and gloominess, a day of clouds and thick darkness, A day of the trumpet and alarm against the fenced cities, and against the high towers. And I will bring distress upon men, that they shall walk like blind men, because they have sinned against the LORD: and their blood shall be poured out as dust, and their flesh as the dung.
Now Zechariah has some use for the phrase:
Zech. 14:1 Behold, the day of the LORD cometh, and thy spoil shall be divided in the midst of thee.
For the first time since Isaiah, TDOL may here be used as a phrase reflecting final judgment. The Lord is king over all the earth (14:9) and the nations all come to observe the Feast of Tabernacles (14:16). Other preterists, though, see this as a reference to 70 AD and believe we are now in a place where the Lord is king over the earth and we observe the feast in the "heavenly Jerusalem". I tend to agree with the latter.
Finally, Malachi uses the phrase:
Malachi 4:5 Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD...
Of course Christian exegetes see this as a prophecy of John the Baptist. Notwithstanding that interpretation, this oracle to the returned exiles speaks of a day that "cometh, that shall burn as an oven; and all the proud, yea, and all that do wickedly, shall be stubble: and the day that cometh shall burn them up, saith the LORD of hosts, that it shall leave them neither root nor branch. But unto you that fear my name shall the Sun of righteousness arise with healing in his wings; and ye shall go forth, and grow up as calves of the stall. And ye shall tread down the wicked; for they shall be ashes under the soles of your feet in the day that I shall do this, saith the LORD of hosts." (4:1-3)
To what does this TDOL refer? TDOL here refers to the destruction of Jerusalem in 70 AD.
Our conclusion: "The day of the Lord" is a general phrase of judgment that can describe the final eschatological judgment of the world, but more often describes any forthcoming day of judgment. What "day" is in mind is determined by context, not merely by the phrase itself.