Why did the Holy Spirit come at Pentecost?

The Holy Spirit came at Pentecost to empower Jewish believers to continue the ministry Christ had accomplished for three years. That ministry included signs and miracles beyond human understanding. The Spirit's coming provided the power these men needed in the interim before the future restoration of Israel's kingdom, as promised by Jesus in Acts 1.

Les explains that in Acts chapter 1, the disciples asked Jesus if He would at that time restore the kingdom to Israel so they could rule the twelve tribes. Jesus did not rebuke them; He said it was not for them to know the times or seasons the Father has put in His own power. But in the interim, until that becomes reality, they would receive power. Les teaches that this is the whole idea of the Holy Spirit's coming on the day of Pentecost: to give these Jewish believers the power to continue the ministry Christ had been accomplishing for three years. That ministry was signs and miracles beyond human understanding, and these men were to have that kind of power.

Acts 1:8

But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.

When was the Holy Spirit going to come upon the disciples?

According to Acts 1:8, the Holy Spirit would come upon the disciples ten days later, on the day of Pentecost. Jesus told them they would receive power after the Holy Spirit came upon them, enabling them to be witnesses in Jerusalem, Judea, Samaria, and to the uttermost parts of the earth.