40 days after the Resurrection Jesus ascended into heaven. Before his Ascension He instructed the Apostles to wait in Jerusalem for the sending of the Holy Spirit. Ten days later, the eleven Apostles, together with the Blessed Virgin Mary, were praying in the Upper Room on Sunday, the Lord's Day. The Holy Spirit descended upon them as tongues of fire. For more information on the Pentecost please see https://www.catholiccompany.com/morning-offering/2019-06-09/