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.