Joel 3:15 - Albert Barnes' Notes on the Bible

The sun and the moon shall be darkened - This may be, either that they shall be outshone by the brightness of the glory of Christ, or that they themselves shall undergo a change, whereof the darkness at the Crucifixion was an image. An ancient writer says ; “As in the dispensation of the Cross the sun failing, there was darkness over all the earth, so when the ‘sign of the Son of man’ appeareth in heaven in the Day of Judgment, the light of the sun and moon and stars shall fail, consumed, as it were by the great might of that sign.” And as the failure of the light of the sun at our Lord’s Passion betokened the shame of nature at the great sin of man, so, at the Day of Judgment, it sets before us the awfulness of God’s judgments, as though “it dared not behold the severity of Him who judgeth and returneth every man’s work upon his own head;” as though “every creature, in the sufferings of others, feared the judgment on itself.”