Moses led the chosen people out of Egypt. It is celebrated as the Passover. The Jews count the days after that for 49 days. Then they celebrate Shovuot = "weeks" = Pentecost. They celebrate the receiving of the ten commandments. The law was given to the people through Moses. In Acts, we read that the Jews were gathering on that day. They were celebrating Shovuot. We call it the Day of Pentecost.
Jesus was crucified during the week of the Passover. After he was resurrected, he spent time with the disciples and others for 40 days before he ascended. Then the Day of Pentecost = Shovuot came, and they received the Holy Spirit! The day of Pentecost was 49 days after Passover. The Day of Pentecost came right after Jesus ascended.
Just like God's presence never left the Jewish people in the desert (by the pillar of fire and the cloud in Exodus, and by the fire and the smoke in Leviticus), Jesus never left the disciples! Even after the resurrection Jesus was with his people, and right after he ascended, the Holy Spirit was sent down, and he has been within us since then.
I had never thought of the time frame until today. It is awesome.