First, the relevant Scripture, from Luke 2:
40 And as the child grew to maturity, he was filled with wisdom; and God's favour was with him.
41 Every year his parents used to go to Jerusalem for the feast of the Passover.
42 When he was twelve years old, they went up for the feast as usual.
43 When the days of the feast were over and they set off home, the boy Jesus stayed behind in Jerusalem without his parents knowing it.
44 They assumed he was somewhere in the party, and it was only after a day's journey that they went to look for him among their relations and acquaintances.
45 When they failed to find him they went back to Jerusalem looking for him everywhere.
46 It happened that, three days later, they found him in the Temple, sitting among the teachers, listening to them, and asking them questions;
47 and all those who heard him were astounded at his intelligence and his replies.
48 They were overcome when they saw him, and his mother said to him, 'My child, why have you done this to us? See how worried your father and I have been, looking for you.'
49 He replied, 'Why were you looking for me? Did you not know that I must be in my Father's house?'
50 But they did not understand what he meant.
51 He went down with them then and came to Nazareth and lived under their authority. His mother stored up all these things in her heart.
52 And Jesus increased in wisdom, in stature, and in favour with God and with people.
Well, recently a thought hit me. This came after a friend asked where Jesus was for those 3 days that His parents were looking for Him. Suddenly the parallel with the 3 days in the tomb struck me. Where was Jesus? He was teaching! In both cases, He was teaching. During this Mystery, He was teaching at the Temple, and during His time in the tomb, He was preaching to the souls of those who had died. I love the foreshadowing!
After I wrote this, I read a great post on the Joyful Mysteries at Shameless Popery. I highly recommend it.