Trinity Sunday is the Sunday following Pentecost. It is a day in which Christians are encouraged to reflect on the ways in which God has made himself known to us. Although God is a mystery Christians believe it is his will that he be made known to some extent. Christians believe he has done so. God has revealed himself as God the Father (the Creator of all things - God "over" us); as God the Son (Jesus, who is both divine and human - the saviour of humanity. God "with" us); and God the Holy Spirit (the power of God who dwells within us – God "in" us). Christians, then, believe in One God in three persons - Father, Son and Holy Spirit.
This is such a central doctrine of the Christian faith that Trinity Sunday was established to remind believers of this truth. The liturgical colour for Trinity Sunday is white.
So central is this doctrine that the rest of the Christian year is understood to be an unfolding of it and so each Sunday following is numbered from Trinity Sunday.