Today the Roman Rite, and much of western Christianity, celebrates Trinity Sunday. What began as a local feast in some parts of the Western church in the Middle Ages, was added to the universal Roman calendar by Pope John XXII (1316–1334), and designated as the first Sunday after Pentecost.
Three folds of the cloth,
yet only one napkin is there,
Three joints in the finger,
but still only one finger fair,
Three leaves of the shamrock,
yet no more than one shamrock to wear,
Frost, snowflakes and ice,
all in water their origin share,
Three Persons in God:
to one God alone we make our prayer.
(An ancient Irish prayer)
The Reverend Doctor Daniel Meeter is pastor of Old First Reformed Church in Brooklyn, New York. He writes:
Unlike most Sundays in our calendar, we are not marking any specific Biblical event, but it makes sense to celebrate the Trinity on the Sunday after Pentecost. On Pentecost God exposed God’s self in the Holy Spirit -- God came among us in the third person of God. Fifty days before that, on Easter, God exposed God’s self the Lord Jesus -- God was among us in the second person of God. The Easter season celebrates the mighty acts of God for our salvation as these actions of two persons, so now that the Season is over, we can put God back together!
We continue with what the reformed Roman calendar refers to in English as "ordinary time." This is ostensibly a faithful rendering, if not a literal one, of the Latin "tempus per annum;" literally, "time during the year." There was a time you might have heard the following Sunday referred to incorrectly as "the Umpteenth Sunday of the Year" or the "Umpteenth Sunday of the Church Year," which it is not, but rather, the Umpteenth Sunday of the regular part of the Church year. Thus, "ordinary" refers to that which is part of the regular order (the words "ordinary" and "order" having the same root) of the year.
Personally, this writer would just as soon they referred to Sundays After Epiphany and/or Pentecost, which some Anglican churches still do, even as they have adopted a more contemporary Book of Common Prayer, and a three-year-cycled lectionary.
And so it goes.