A.T. Robertson 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)

When they had fulfilled the days (τελειωσαντων τας ημερας). Genitive absolute again, but aorist participle (effective aorist). "The days" may mean the full seven days (Exodus 12:15f.; Leviticus 23:6–8; Deuteronomy 16:3), or the two chief days after which many pilgrims left for home.

As they were returning (εν τω υποστρεφειν αντους). The articular infinitive with εν, a construction that Luke often uses (1:21; 2:27).

The boy, Jesus (Ιησους ο παις). More exactly, "Jesus the boy." In verse 40 it was "the child " (το παιδιον), here it is "the boy" (ο παις, no longer the diminutive form). It was not disobedience on the part of "the boy" that made him remain behind, but intense interest in the services of the temple; "involuntary preoccupation" (Bruce) held him fast.