Theophylact of Ohrid

Jesus goes to Jerusalem together with His parents, in order to show in everything that He neither opposes God nor resists the requirements of the Law. When they had fulfilled the days, that is, the seven days of Passover, He "remained... in Jerusalem."

Commentary on Luke ↗

Ancient Greek Expositor

The feast having been celebrated, while the rest returned, Jesus secretly tarried behind. As it follows, And when they had fulfilled the days, as they returned, the child Jesus tarried behind in Jerusalem; and his parents knew not of it. It is said, When the days were accomplished, because the feast lasted seven days. …

Catena Aurea by Aquinas ↗

Bede

And his parents did not know. But assuming that he was in the company, they went a day's journey, and sought him among relatives and acquaintances, and not finding him, they returned to Jerusalem searching for him. Someone might ask how the Son of God, reared with such care by his parents, could have been forgotten and left behind as they went away. …

On the Gospel of Luke ↗

Bonaventure

And when the days were completed, as they returned, namely to their own homes, according to that passage in Judith chapter sixteen: "When the feast was celebrated, each one returned to his own home." And while all returned, Jesus nevertheless did not: whence it is added: The boy Jesus remained in Jerusalem, and thus alone, without the company of his parents. …

Commentary on Luke, Chapter 2 ↗