Jesus' instruction at the time of His ascension was, "not to depart from Jerusalem, but to wait for the promise of the Father...before many days you shall be baptized with the Holy Spirit." And the Apostles did just that: "All these with one accord devoted themselves to prayer, together with the women and Mary the mother of Jesus, and with his brethren" (Acts 1:4-5, 14). That was how they spent the nine days between Jesus' ascension and the descent of the Spirit on Pentecost.
Of what did their prayer consist? Petition, combined with a great deal of meditation (the "soul" of the Rosary) - thinking and rethinking the things Jesus had said to them, the actions and miracles they witnessed, the meaning of His death, resurrection, and ascension. It consisted of reflecting upon Scripture; when Jesus appeared to them on the night of His resurrection, He had "opened their minds to understand the scriptures...the law of Moses, the prophets and the psalms" and how they had been fulfilled in Him (Luke 24:44-45). And this meditation was being done in the presence of Mary. She was engaged in it with them. As John Paul II pointed out so beautifully:
Mary lived with her eyes fixed on Christ, treasuring his every word: “She kept all these things, pondering them in her heart” (Lk 2:19; cf. 2:51). The memories of Jesus, impressed upon her heart, were always with her, leading her to reflect on the various moments of her life at her Son's side. In a way those memories were to be the “rosary” which she recited uninterruptedly throughout her earthly life (Rosarium Virginis Mariae, 11).The Apostles spent nine days engaged in this with her, making the Church's first novena. We can see the fruits that emerged - Peter's move to replace the office left vacant by Judas' defection emerged from his reflection upon the Psalms (Acts 1:20) and then the explosion of Scriptural insights he unleashed up the crowd at Pentecost! (Acts 2:16-41) Isn't it likely that the Holy Spirit had been bringing key points of that first sermon to Peter's consciousness throughout the nine days of prayer?
When we today pray the Rosary, when we recite the Hail Mary while meditating upon the events recounted in the gospels and Acts (the fifteen Mysteries), we enter into the Apostles' experience. "With the Rosary, the Christian people sits at the school of Mary and is led to contemplate the beauty on the face of Christ and to experience the depths of his love" (Rosarium Virginis Mariae, 1). And by doing this regularly, daily, our souls grow and become progressively docile to the movement of the Holy Spirit. We receive not one, but several Pentecosts as our eyes open up onto new spiritual vistas and we find ourselves acting with a freedom and strength we imagined ourselves unable to attain. And rightly so - these things can only take root in souls that have been broken up and seeded, over time, through prayer. These souls are made ready for that moment when the Living Water rains down and causes the new life to burst forth out into the open.
We won't see Pentecost without it. Jesus knows how we are made, and He knows how to "remake" us in His image; that was why He sent the Apostles back to the upper room. They needed to spend that time in prayer, in meditation...in the company of His Mother. My friends, almost two thousand years may have passed, but the prescription remains the same.