We know nothing about the prophet "Joel, son of Pethuel," aside from what we can glean from the writings that appear in his book.
It is not certain exactly when the prophet lived. Because of certain linguistic and theological features of the book, many Old Testament scholars have concluded that Joel prophesied after the people's return from the Babylonian exile. But the canonical placement of Joel between Hosea and Amos bears witness to a traditional view that dates his ministry earlier.
Even the historical crisis that occasioned Joel's prophetic messages is obscure. The prophet describes the land as falling under the shadow of an invasion of a "locust" army (1:4), which devastated Judah's crops, leaving land, animals, and populace groaning in travail. But scholars disagree about whether the "locust" should be taken literally (and thus that Joel was responding to an ecological plague), or be understood as a metaphor for a hostile human army (and thus that Joel was responding to a military crisis).
Whichever view one takes regarding the historical occasion for the book, the theological theme is clear. The prophet announced that the crisis of locust was no mere accident, but the hand of God. The invading army of locust was God's punishment (it is described as "my great army" in 2:25). Even more telling, Joel announced that the present historical crisis was a foretaste of a less than desirable eschatological feast to come - "the day of the Lord" (1:15). In response, Joel bid the people to repent and throw themselves on God's mercy: "Who knows whether he will not turn and relent, and leave a blessing behind him" (2:14).
The Book of Joel can be divided neatly into two parts. In 1:1-2:17, the crisis is described as God's judgment and the people are called upon to repent. The pericope for Ash Wednesday falls at the end of this section. The rest of the book contains the prophet's announcement of the advent of the Lord's mercy in new and surprising ways.
Unlike the prophets Amos, Micah, or Isaiah, Joel did not emphasize repentance as turning away from evil and toward a life of justice. Rather, Joel emphasized repentance as turning to the Lord in worship: "Blow the trumpet in Zion; sanctify a fast; call a solemn assembly" (2:15). Joel does not focus on justice, name specific sins, or single out specific people or castes of people for their sins. Rather, Joel bids the people to turn toward God in repentance and worship.
Similar to Amos (cf. Amos 5:18-20), Joel used the term "day of the Lord" not to refer to the end of time, but to the time when God would act within history. The term "day of the Lord" probably began as a reference to Israel's major religious festival, the festival of Tabernacles - "the day of the festival of the Lord" (Hosea 9:5).
At major religious festivals, the "trumpet" (Hebrew shophar, the horn of an animal used as a wind instrument) was sounded. The trumpet's call could signal the year of Jubilee (Leviticus 25:9), the festival of the new moon (Psalm 81:3), or the processional going up of the Ark of the Covenant (Psalm 47:5). But the real point of sounding the trumpet is to announce the advent of the Lord (Exodus 19:16, 19; 20:18; etc.).
Joel's cry to "blow the trumpet in Zion, sound the alarm on my holy mountain" is the announcement that the day of the Lord "is coming, it is near" (2:1). And like the Amos, Joel was announcing that the Lord's coming was not the good news the people had expected, but bad news. The people of Israel looked forward to the day of the Lord as a child today looks forward to Christmas. They thought it would be the day when the Lord would act within history to deliver Israel from her enemies, the day when the Lord would defeat Israel's foes. Not so fast, announces Joel! It is "a day of darkness and gloom" (2:2).
And then comes the surprise in Joel's message - at least when compared to Amos 5, Micah 6, or Isaiah 2. The prophet calls on the people to a worship service of repentance: "return to me with all your heart, with fasting, with weeping, and with mourning" (2:12).
From a Christian, third-millennial perspective, a call to repentance probably seems rather ho-hum, a been-there-heard-that-before kind of theological move. But Joel's perspective on repentance and the role of worship in repentance is a fresh, new, and exciting word. And to score this point, Joel rhetorically exploits the link between the concepts of "the day of the Lord" as God's coming to judge and "the day of the Lord" as the festival day.
Joel repeats the cry to "blow the trumpet" (2:15). But this time, rather than continuing with "sound the alarm," as in 2:1, he proceeds with "sanctify a fast." Joel believes that there is connection between worship rituals and genuine repentance - a link between "rending your heart" and "gathering the people."
Where is that link to be found? What is its nature? It is in God's character. Joel has confidence that ritual repentance can change the course of the history of God's people because he believes the old confessional formula:
[God] is gracious and merciful,
Slow to anger, and abounding in steadfast love,
And relents from punishing. (2:13)
Joel believes, and so Joel proclaims: God's character is "faithful" (perhaps a better translation in this context of the Hebrew hesed than "steadfast love"). And because God's character is to be faithful, the horizon, dark and gloomy with storm clouds of judgment as night falls, can now shine crisp and clear with the Lord's favor, when morning dawns.