(Luke 2:39-50)
According to God's law, every male was required to go to Jerusalem three times a year for the Feast of Unleavened Bread (Passover), the Feast of Weeks, and the Feast of Tabernacles. However, only the day of Passover was strictly observed. Entire families went up to Jerusalem and an estimated 60,000 to 100,000 visitors packed themselves into a town of 25,000. Large caravans of people traveled together for companionship and safety.

When Jesus was 12 years old, His family went to Jerusalem for the feast as was their custom. At 12 years old, a boy was considered almost an adult. When large groups traveled together, the women and children were at the front of the caravan and the men traveled at the back. Jesus could have been in either group, so it is not surprising that He was not immediately missed.

When Mary and Joseph realized that He was not with their group, they returned to Jerusalem looking for Him and they finally found Him in the temple courts, sitting among the teachers. He was listening to them and questioning them. The rabbis were amazed at His understanding and His answers.

Jesus' response to His parents searching for Him is the first mention of Jesus' awareness that He was God's Son. However, He did not reject his earthly parents. He went back to Nazareth with them and continued in obedience.
(Life Application Bible, New International Version, Tyndale House Publishers, Inc., Wheaton Illinois, and Zondervan Publishing House, Grand Rapids, Michigan, 1991, 1794.) (D. Guthrie and J. A. Motyer eds. The New Bible Commentary: Revised. Wm. B. Eerdmans Publishing Co., Grand Rapids, Michigan, 1970, 894.)

Hebrew Law and Custom

(Luke 2:39-50)
The Feast of the Passover, or Unleavened Bread, was one of three great annual festivals of Israel. It was observed in remembrance of the Lord delivering the Jews from the last plague which struck the Egyptians, and Israel's ultimate delivery from bondage (Exodus 11-12). The festival included the observance of the day of Passover followed by the seven-day Feast of Unleavened Bread.
(Merrill F. Unger, Unger's Bible Dictionary, Moody Press: Chicago, 1966, 352-3.)

(Luke 2:44)
Traveling in a large caravan, as was common, it would have been normal for Mary and Joseph to be unconcerned, thinking Jesus was among relatives in the group.
(Marshall, 127)

Joseph and Mary's home was in Nazareth (cf. Luke 2:39), but being from the family of David, Joseph's ancestral home was Bethlehem. This practice of registering in a town where one does not currently reside is not unknown in ancient times.
(Bible Knowledge Commentary, 208; Marshall, I. Howard, The Gospel of Luke, Eerdmans, Grand Rapids, 1978,101-102.)