The second half of Acts 1 deals with a period of waiting on the part of the disciples prior to the coming of the Holy Spirit at Pentecost, which is described in chapter 2. It lasted ten days. We know that it was ten days because the Holy Spirit came at Pentecost. Pentecost refers to the Feast of Weeks, which was held fifty days after Passover. Since the Lord was taken back to heaven forty days after the resurrection, there must have been a ten-day period in which the disciples waited in Jerusalem.