The covenant is the major underlying theme in Joel. The day of Yhwh locust judgment that comes upon Israel is a consequence of Israel’s violation of the Mosaic covenant. However, the Mosaic covenant also predicted the coming new covenant for repentant Israel. The new covenant would result in a reversal of judgment. In Joel the revelation of the new covenant is extended to indicate that Gentiles will be caught up in its promises as God says that the Spirit will be poured out on all flesh. The gift of the Spirit is at the heart of the new covenant. The application of all the other promises of the new covenant depends on the gift of the Spirit.
The Day of Yhwh is a day in which Yhwh intervenes to judge humans for their sins. The Flood would be a day of Yhwh. As would be Joel’s locust plague, the conquest of Judah by Babylon, and the fall of Jerusalem to Rome in AD 70. But all of these historical days of Yhwh point forward to the eschatological day of Yhwh, when the Lord returns to judge all mankind for sin. The New Testament refers to this day both as the day of the Lord and as the day of Christ, since Christ is the one to whom the Father has committed all judgment. If the day of Yhwh is the day of Christ, then Christ must be Yhwh. There is also a positive aspect to the day of Yhwh in which God’s people and the entire earth experience restoration.
While there are events that before Obadiah and Joel that we can now call days of Yhwh (like the Flood), the terminology was first introduced in Obadiah 15 and then developed extensively in Joel (who includes both the judgment and restoration aspects). Chronologically, these are the first of the writing prophets, and within the Book of the Twelve Joel is placed second. Thus, this foundational prophetic theme is introduced early and developed by the rest of the prophets.
Repentance is the only proper response to the day of Yhwh. For those within a day of Yhwh it is the only path towards forgiveness and restoration. For those with the day of Yhwh looming near, it is the only means by which to avert God’s judgment. The book of Joel was written to spur people to repentance in the face of the day of Yhwh judgment that his people then faced and that all of his readers would face if they did not repent. Joel is a book of judgment, but it is also a book of restoration. Judgment is the first word, but it is not the last word. For the repentant, Yhwh will restore the years the locusts have eaten. And the book closes with the land flowing with abundance.