Christians around the world celebrate the Day of Pentecost. It’s often remembered as the church’s birthday. Before Jesus ascended into heaven, he promised that the Holy Spirit would come upon those who believed in his resurrection.

Three thousand people were gathered in Jerusalem when the Spirit came upon them. They were so touched by the Holy Spirit that they began to worship regularly, especially on Sundays, the day of Christ’s resurrection.

To symbolize the “tongues, as of fire” (Acts 2:3, NRSV) that came upon the people present at Pentecost, liturgical churches display the color red in their worship services on Pentecost Sunday. Many people attending worship wear red on that day.

Common Lutheran Ideas
Explanation provided in link.

For more information about Lutheranism, please see the ELCA website