It's a later addition to the liturgical calendar, somewhere in the 11th or 12th century. Few pastors consider this a favorite preaching topic, in fact some pastors call this their least-favorite Sunday (a good Sunday for a new associate or guest preacher). Other than the baptismal formulation and an occasional benediction, we Protestants and evangelicals do not use trinitarian language very much. I've noticed over the years that the language of contemporary worship songs is very Christocentric and cross-oriented, but not very trinitarian. Why?
Different denominations have reputations for emphasizing one person of the Godhead: Presbyterians love the sovereignty of God and Covenanters love the warm-hearted tenderness of Jesus the good shepherd, and our Independent, Calvary Chapel/Vineyard churches are totally energized by the presence of the Holy Spirit. What's with this? How does an everyday Christian keep a trinitarian theology?
In teaching confirmation, I've always turned students loose in the sanctuaries on a hunt for trinitarian hints in the architecture: 3 window panes, 3 steps up to the chancel from the nave, the pulpit/font/table, 3 sections of seating, 3 doors of entry. Look for the 3's I say. And then when we go to visit a Roman Catholic Church, they go wild with the architectural reinforcement of the trinity.
This week's epiphany for me is that the Trinity is God's way of telling that true life, abundant living is always in a dynamic community, just like Father, Son and Holy Spirit, bonded by grace, love and fellowship. God models for us the way we are to live together; creatively distinct and diverse, yet loving and delighted with each other.