Had fulfilled the days - The days of the Passover. These were eight days in all - one day for killing the paschal lamb, and seven days for the observance of the feast of unleavened bread, Exodus 12:15; Leviticus 23:5-6.

Had fulfilled the days - Eight days in the whole: one was the passover, and the other seven, the days of unleavened bread. See on Matthew 26:2 (note).

And when they had fulfilled the days,.... The seven days of the fear of unleavened bread, for so many days that feast was observed; and though it was not absolutely necessary, and obligatory upon them to stay all that time at Jerusalem, yet Mary and Joseph seem so to have done, as did the more religious and devout persons:
as they returned; at the time when they were going from Jerusalem home again:
the child Jesus tarried behind in Jerusalem; being desirous of hearing the discourses of the doctors about the sense of the Scriptures, the meaning of the laws, and the traditions of the elders, and of conversing with them:
and Joseph, and his mother, knew not of it; of his intention to tarry longer, nor of his design in so doing: he did not ask leave of them, since his stay was about an affair of his heavenly Father's; and therefore this action of Christ is not to be drawn into an example, or precedent for children, to act without consulting, or asking leave of their parents. They had no notion at all of his staying behind them, nor any suspicion of it; nor did they miss him for a considerable time; which might be owing to the large numbers that went in company together, so that they could not tell but that he was in the crowd, though they did not see him; or to the men and women travelling in separate companies, as is thought; so that Joseph might think he was with Mary, and Mary might conclude he was with Joseph, till they came to the end of their first day's journey, when they came together, and then missed him.

as they returned--If the duties of life must give place to worship, worship, in its turn, must give place to them. Jerusalem is good, but Nazareth is good, too; let him who neglects the one, on pretext of attending to the other, ponder this scene.
tarried behind . . . Joseph and his mother knew not--Accustomed to the discretion and obedience of the lad [OLSHAUSEN], they might be thrown off their guard.

Fulfilled the days. The seven of the passover week.
Jesus tarried behind. Luke neither tells us that Jesus remained behind at Jerusalem intentionally, nor that Joseph and Mary lost sight of him through want of necessary care. A circumstance must here have been omitted, and we may safely suppose that Joseph and Mary joined their fellow-travelers in the persuasion that Jesus, who knew of the time and place of departure, was among the younger ones.

The child Jesus - St. Luke describes in order Jesus the fruit of the womb, Luke 1:42; an infant, Luke 2:12; a little child, Luke 2:40; a child here, and afterward a man. So our Lord passed through and sanctified every stage of human life. Old age only did not become him.