The Trinity Doctrine
The doctrine of the Trinity, on which Trinity Sunday is based, holds God to be triune, having the quality of three-in-oneness. In this view, God is a single being existing always and forever as a perfect communion of three distinct persons, which are commonly called the Father (the Source of Eternal Majesty), the Son (the Eternal Word or Logos), and the Holy Spirit.
Some Christians contend that one of the three (often the Father) is the truly supreme God, whereas the Son and the Holy Spirit are considered aspects of the former's divinity. Others believe the three parts of the Trinity are different ways that an eternal God has chosen to manifest himself to humanity. However, most Christians now hold that the Trinity refers to three persons in one substance. From this standpoint, the answer to the question What is God? is infinite, endless divinity, while the answer to the question Who is God? is the Father, Son, and Holy Spirit.
Roman Catholic Traditions
For the Roman Catholic Church, Trinity Sunday is a day of transition between the seasons of Pentecost, with its reflections on what the Godly triune has done to accomplish the salvation of mankind, and Ordinary Time, wherein the focus of the Church swings to how people should respond to the infinite love of the Almighty. The color of the vestments and sanctuary for the day is typically white, signifying the pure light of God's love. Prior to the Second Vatican Council, the festival marked the end of a three-week moratorium on Church weddings for the Pentecost season.
For many Christians, Trinity Sunday is an important time of reflection. Between the seasons of Pentecost and Ordinary Time, the celebration allows space for the contemplation of the triune nature of God and the different ways his infinite love is expressed to humanity.