Then returned they unto Jerusalem
Jesus instructed the apostles to wait in Jerusalem until the Spirit clothed them with power (Luke 24:49).
from the mount called Olivet,
The Mount of Olives, a hill to the east of Jerusalem across the Kidron Valley, was near the village of Bethany and the location of the Lord’s ascension (Luke 24:50).
which is from Jerusalem a sabbath day’s journey.
Under Jewish tradition, no one should walk more than 2,000 cubits on the Sabbath. God never prescribed such a rule, but 2,000 cubits was the distance the Israelites’ were to keep between them and the ark of the covenant when they wandered the wilderness (Josh. 3:3-4).
And when they were come in, they went up into an upper room, where abode
Once the disciples returned to Jerusalem, they went into an upper room of a house that may have been their hiding place prior to Jesus’ resurrection (John 20:19). It could be the same room where they ate the last Passover meal with Christ (Mark 14:14-16). It must have been a large room since it accommodated 120 people (Acts 1:15). They stayed in that place but often walked to the temple to worship God (Luke 24:52-53).
both Peter, and James, and John, and Andrew, Philip, and Thomas, Bartholomew, and Matthew, James the son of Alphaeus, and Simon Zelotes, and Judas the brother of James.
The eleven remaining apostles were present. Judas Iscariot, the twelfth, was dead (Matt. 27:5).
These all continued with one accord in prayer and supplication,
There was spiritual unity among those who gathered in the upper room as they devoted themselves to continual prayer. Though Jesus was gone, the disciples knew they could still communicate with Him through prayer (John 14:13-14).
with the women, and Mary the mother of Jesus,
The women present with Jesus’ mother likely included Mary Magdalene, Mary (Clopas’ wife), Mary and Martha (Lazarus’ sisters), and Salome (John 12:1-3; Matt. 27:55-56; Mark 15:40; John 19:25). Perhaps the Lord’s half-sisters were there as well (Mark 6:3).
and with his brethren.
Jesus had four half-brothers including James, Joseph, Judas, and Simon (Mark 6:3). Not long before, they were unbelievers who may have been converted by the sight of the resurrected Christ (John 7:5; 1 Cor. 15:7).
And in those days Peter stood up in the midst of the disciples, and said, (the number of names together were about an hundred and twenty,)
A relatively small group of 120 disciples would soon multiply, exploding across the face of the earth (Acts 6:7; 9:31; 12:24; 16:5; 19:20).