When we look at Church Year, we see that the primary focus is on the life of Jesus. We begin the year with the lead up to Jesus' birth. Shortly after, we remember Jesus' ministry as we prepare ourselves to celebrate His death and Resurrection. That leads us to the Day of Pentecost, when Jesus has left this world and sent the Holy Spirit in the hearts of the Apostles so that they may spread the Good News to the ends of the earth.
Jesus rightly is the focus of most of our calendar, as He is the Savior of the world. The Holy Spirit gets a day of celebration with Pentecost. While the Father does not have a specific day of celebration in the church calendar, the vast majority of our prayers are addressed directly to God the Father Almighty.
In response to the desire for a day to celebrate the unity of God, some Christian communities began to mark off one Sunday for the Trinity. The idea grew in popularity over the centuries, leading to our celebration of Trinity Sunday today on the Sunday after we celebrate the end of Jesus' physical ministry among us on earth.
The beauty is that we celebrate the unity of God in unity on one Sunday. While every Sunday is a celebration of God, this Sunday remembers God's unity in the Three Persons we have come to experience our Lord on this world.
To learn more, check out:
Alexander, Neil. Creating Liturgical Time. New York: Church Publishing, 2014