... continued from yesterday
Jesus appears to his followers for forty days after his resurrection before he ascends to heaven. Many churches today celebrate this period in their worship calendars. Some even set aside the Sunday after the forty days as "Ascension Sunday."
We shouldn't think that Jesus was with them continuously for this forty day period. Paul gives us a more detailed sense of this time in 1 Corinthians 15. Jesus would appear now to one person, now to another. By Paul's reckoning, Jesus appeared first to Peter, then to the other ten remaining apostles (1 Cor. 15:5). 
Paul also mentions an appearance to over five hundred people at one time (15:6). Was that the day of the ascension? Was that the Great Commission appearance in Galilee (Matt. 28:16-17)? Many people assume that Matthew's Great Commission appearance is the same appearance where Jesus ascends, but this is not the case. Matthew's Great Commission is in Galilee, while Acts' ascension is in Jerusalem.
Some of the appearances to apostles like James, the Lord's brother, may also have taken place during the forty days before the ascension. Or they may have taken place afterwards, like the appearance to Paul. Although it may make us a little uncomfortable, we simply do not have enough information to nail down the precise details for sure.
Then you have the possibility that Luke has used some creativity in the way he has presented the story. For example, if you look at the ending of Luke, you could easily think that Jesus rose from the dead and ascended to heaven on the same day! Jesus meets the two men on the road to Emmaus "that same day" (Luke 24:13). They returned "at once" to Jerusalem (24:33). And while they discuss these things with the disciples, Jesus appears to the disciples for the first time, as in Matthew and John, seemingly on the evening of his resurrection (24:36).
If all we had were Luke, we would not see forty days inserted between Luke 24:49 and 24:50. In Luke, it sounds like Jesus explained things to them for a while, led them out toward Bethany, and then ascended to the skies. It reminds us that while we would prefer the narratives of the Bible to fit together neatly, it is not always that easy. For example, Luke gives us no hint in his resurrection stories that Jesus appeared to the disciples in Galilee. Meanwhile, the two oldest gospels, Mark and Matthew, seem to indicate that the most central appearances were in Galilee.
Acts is the second volume of Luke, while we do not have second volumes for Matthew, Mark, or John. It is sobering from a historical perspective to realize that, if the other gospel writers had written second volumes, they would likely differ from each other as much as Luke differs from the other gospels. If we were to discover a second volume to Mark, it would no doubt change a number of easy assumptions we now have.
The four gospels certainly already affect our assumptions this way. Having Matthew changes our assumptions about Mark, and having John changes our assumptions about all the rest. In the end, we have to get comfortable with some degree of uncertainty about how things happened exactly...
 Did the tradition simply omit the appearance to Mary Magdalene in John 20:11-18? Was it not known?