Adam Clarke on Joel 2:17

Let the priests - weep between the porch and the altar - The altar of burnt-offerings stood before the porch of the temple, 2Chr 8:12, and between them there was an open space of fifteen or twenty cubits. It was there that the priests prostrated themselves on such occasions. It was into this place that the priests brought the sacrifice or victim of atonement; and where the high priest laid his hands on the head of the victim confessing his sins.

Let them say - The following was the form to be used on this occasion, "Spare thy people," etc. And if this be done with a rent heart, etc., "then will the Lord be jealous for his land, and pity his people," 2Chr 8:18. He will surely save, if ye seriously return to and penitently seek him.