Pentecost Sunday (this year happens to fall on the 19th May) in the Christian Calendar commemorates the day when the Holy Spirit descended upon the Apostles.
It occurs 50 days after Easter (as in Acts 2:1-13). This day became significant for Christians because, seven weeks after the resurrection of Jesus, during the Jewish celebration of Shavuot/Pentecost, the Holy Spirit was poured out upon His first followers, and as a result the disciples of Christ were empowered for their mission. So, this is also to celebrate the birth of the Church.
The liturgical colors generally use red on Pentecost as a symbol of power and fire of the Spirit and the blood of the martyrs, which is the seed of the Church.