The Study of Acts 1:13

Acts 1:13
When they had entered the city, they went up to the (a)upper room where they were (b)staying; that is, (c)Peter and John and James and Andrew, Philip and Thomas, Bartholomew and Matthew, James the son of Alphaeus, and Simon the Zealot, and Judas the son of James. 

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Gathering in the upper room

Ruins of a large upper room

​v.13 (a)  upper rooms were generally larger in size because of the structure of the houses back then. The lower levels had more walls and smaller rooms to support the weight of the second and third stories above. Upper room rooms like this were usually on the third floor and reached by outside steps. They were often used as dining rooms, as study places for students, or were sublet and rented as I believe is the case with the apostles.
This might be the same upper room  that  the disciples and Jesus had their Last Supper, Lk 22:7-13; it is interesting to think that maybe Jesus rented this room not just for the night, but for the next couple of months in anticipation of the events that would follow.

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v.13 (b)  the disciples were obeying the Lord’s command to wait in the city until they receive the Holy Spirit and are clothed with power from on high, Lk 24:49, Acts 1:4.
 
v.13 (c)  this is the same list of apostles as in Lk 6:14-16 with the exception of course of Judas Iscariot. There is however a change in the order and placement of the names. In the Bible, it is very common that the order of a name in a list, like a genealogy, is not necessarily the order from oldest to youngest, but instead the order is made by importance.
We see that here, as compared to the list in Lk 6-14-16. In the list in Acts, Peter and John have been elevated to a high standing as leaders among the apostles, thus they are listed first instead of with their brothers as we see in the list from Luke.
 
The amount of persecution they were experiencing is more than can be imagined. The Pharisees wanted to kill them and Romans wanted to arrest them for causing a disturbance.
They had just spent forty days on and off with Jesus after the resurrection, but they did not yet have the power of the Holy Spirit to give them confidence and boldness. My guess is they were following the Lord’s command to stay in Jerusalem, but still being very discreet and quiet; probably still behind locked doors.
Jesus had spent the last 40 days opening their eyes to the scriptures; showing them that His suffering that had just taken place was, in fact written hundreds of years before as prophecy. He was building their faith and confidence in the word; giving them insight and understanding to His plan. 

So that when the power of the Holy Spirit would come, they would already have the word in their hearts to boldly preach the gospel of Christ, even unto persecution, arrest, tribulation and even death.