Luke 2:41-46

Luke 2:41 (KJB)
Now his parents went to Jerusalem every year at the feast of the passover.

In first century Palestine, as many males as possible would come to Jerusalem for the Passover. (Deu 16:16 KJV) Three times in a year shall all thy males appear before the LORD thy God in the place which he shall choose; in the feast of unleavened bread, and in the feast of weeks, and in the feast of tabernacles: and they shall not appear before the LORD empty: It was commanded in the law that all males were to appear before the Lord. So Joseph being an obedient Jew, made the trek to Jerusalem from Nazareth. By this time the Passover and feast of Unleavened Bread had become one feast which lasted 8 days.

Luke 2:42 (KJB)
And when he was twelve years old, they went up to Jerusalem after the custom of the feast.

During Bible times, boys who reached the age of twelve became sons of the law, we know it as bar mitzvah. At that time they became adult members of the Jewish community and were expected to obey the law. This feast was probably the time of the bar mitzvah of Jesus. It is not certain as to whether this was His first visit to the temple but since Joseph sought to obey the law and attend the many feasts, He probably was here before.

Luke 2:43 (KJB)
And when they had fulfilled the days, as they returned, the child Jesus tarried behind in Jerusalem; and Joseph and his mother knew not of it.

When the feast was over and it was time to return to Nazareth, Jesus had tarried in Jerusalem but Joseph and His mother did not know this. Sometimes the people traveled in a group and the men and women would travel in separate groups. It is also possible that they thought Jesus might have been with some of the other children in another wagon and that is why they did not know He was missing.

Luke 2:44 (KJB)
But they, supposing him to have been in the company, went a day's journey; and they sought him among their kinsfolk and acquaintance.

Jerusalem to Nazareth was about 65 miles (104 km) and was about three days journey. This would mean that they were out of Jerusalem about 20 miles (35 km) when they began to search for Him among the caravan. They did not think that Jesus would stay behind and that He would have known when they were leaving.

Luke 2:45 (KJB)
And when they found him not, they turned back again to Jerusalem, seeking him.

Mary and Joseph then went back to Jerusalem and began the diligent search for their Son. No doubt they were worried about Him especially with all of the prophecies surrounding Him. So this trek back to Jerusalem would have extended their trip by two days and now they would probably have to return to Nazareth without the company of others, which is the way they traveled in those days for fellowship and protection.

Luke 2:46 (KJB)
And it came to pass, that after three days they found him in the temple, sitting in the midst of the doctors, both hearing them, and asking them questions.

It seems that they had looked in every quarter of Jerusalem for Jesus and finally after three days of searching they found Him in the temple. If you notice that He was sitting in the temple. In those days when someone was teaching, they sat while they taught. The listening to the teachers and then asking questions were the typical activity for a student. The temple was not only a place for sacrifices and worship but was also the central place for teaching and exhortation. The rabbinic instruction normally took place in the outer courts or rooms of the temple and it was here that Mary and Joseph had found Jesus.