Albert Barnes Commentary Luke 2:43

"and when they had fulfilled the days, as they were returning, the boy Jesus tarried behind in Jerusalem; and his parents knew it not;" — Luke 2:43 (ASV)

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.