Traditionally, the book of Jonah is read, in its entirety, during the synagogue service on Yom Kippur afternoon. Nahum Sarna explains why (1990):
The major themes of the book are singularly appropriate to the occasion—sin and divine judgment, repentance and divine forgiveness.
What is remarkable is that the work is not at all about [the people] Israel. The sinners and penitents and the sympathetic characters are all pagans, while the anti-hero, the one who misunderstands the true nature of the one God, is none other than the Hebrew prophet. He is the one whom God must teach a lesson in compassion.
It is precisely these aspects of this sublime prophetic allegory, and in particular the subthemes of the book, that inform Yom Kippur. . . . [The book’s] universalistic outlook; its definition of sin as predominantly moral sin; its teaching of human responsibility and accountability; its apprehension that true repentance is determined by deeds and established by transformation of character (Jonah 3:10), not by the recitation of formulas, however fervent; its emphasis on the infinite preciousness of all living things in the sight of God (Jonah 4:10–11); and, finally, its understanding of God as “compassionate and gracious, slow to anger, abounding in loving-kindness” (Jonah 4:2)—all these noble ideas of the book of Jonah constitute the fundamentals of Judaism and the quintessence of Yom Kippur.