Contradiction with Matthew 6:16-18
Joel 2:15 calls for a public fast, while Matthew 6:16-18 advises fasting in secrecy.
Matthew 6:16-18: Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast. Verily I say unto you, They have their reward.
Contradiction with Amos 8:10
Joel 2:15 promotes a solemn assembly, whereas Amos 8:10 describes God turning feasts into mourning as a punishment.
Amos 8:10: And I will turn your feasts into mourning, and all your songs into lamentation; and I will bring up sackcloth upon all loins, and baldness upon every head; and I will make it as the mourning of an only [son], and the end thereof as a bitter day.
Contradiction with Isaiah 58:5
Joel 2:15 emphasizes a communal call to fasting, yet Isaiah 58:5 criticizes mere ritualistic fasting without sincere change.
Isaiah 58:5: Is it such a fast that I have chosen? a day for a man to afflict his soul? [is it] to bow down his head as a bulrush, and to spread sackcloth and ashes [under him]? wilt thou call this a fast, and an acceptable day to the LORD? [a day...: or, to afflict his soul for a day?]
Contradiction with Hosea 6:6
Joel 2:15 highlights an outward act of fasting, whereas Hosea 6:6 prioritizes mercy and knowledge of God over sacrifices.
Hosea 6:6: For I desired mercy, and not sacrifice; and the knowledge of God more than burnt offerings.