The Descent of the Holy Spirit at Pentecost
The Third Glorious Mystery of the Rosary is the Descent of the Holy Spirit on Pentecost Sunday, ten days after the Ascension.
At the top of the window is a symbol of the tongues of fire representing the Holy Spirit descending upon the apostles. (see Acts 2:1-21)
The dove is a symbol of the Holy Spirit. There is no mention of a dove in Acts, but we associate a dove with the Holy Spirit because of the story about Jesus' baptism: "After Jesus was baptized, he came up from the water and behold, the heavens were opened [for him], and he saw the Spirit of God descending like a dove [and] coming upon him" [Matthew 3:16].
The rest of the window depicts Mary with the apostles in the upper room in Jerusalem, the site of the Holy Spirit descending upon the apostles at Pentecost.