Trinity Sunday, kept on the Sunday after Pentecost, is unusual among the great feasts in celebrating not an event in the life of Christ but a doctrine: the confession of one God in three persons, Father, Son and Holy Spirit. Its placement is deliberate. The temporal cycle from Advent through Pentecost rehearses the acts by which, in Christian belief, the triune God is made known, from the sending of the Son to the sending of the Spirit; Trinity Sunday gathers the whole sequence into the doctrine it is held to reveal, and the long "ordinary" weeks of the year that follow are counted from it.
A votive office of the Trinity circulated in the medieval West well before the feast was general; Pope John XXII extended it to the whole Latin church in 1334. In Anglican use the Sundays of the second half of the year were long numbered "after Trinity", and the English legal and academic Trinity term takes its name from the feast. It is one of the few feasts that gives a technical point of theology a fixed day in the calendar.