Trinity Sunday

Trinity Sunday, first Sunday after Pentecost, observed as a feast of the Trinity. It was an innovation in medieval England and spread through the Western Church in the 14th cent. The Sundays until Advent are counted from either Pentecost or Trinity.

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Trinity Sunday

Between May 17 and June 20; first Sunday after Pentecost in the West and Monday after Pentecost in the East

Trinity Sunday differs from other days in the Christian calendar in that it is not associated with a particular saint or historic event. Instead, it is a day that celebrates the central dogma of Christian theology: that the One God exists as three persons with one substance—as the Father, the Son, and the Holy Spirit. The idea of a festival in honor of the Trinity was first introduced by Stephen, bishop of LiÅge, Belgium, in the 10th century. But it took several more centuries for a feast in honor of so abstract a concept to find its way into the church calendar. It became popular in England perhaps because of the consecration of Thomas à Becket on that day in 1162, but it wasn't until 1334 that it became a universal observance decreed by Pope John XXII. The day after Trinity is sometimes referred to as Trinity Monday.
Tradition has it that St. Patrick of Ireland used a shamrock as a symbol of the "three-in-one," triune God.

SOURCES:
DaysCustFaith-1957, p. 164
DictWrldRel-1989, p. 768
FestSaintDays-1915, p. 128
OxYear-1999, p. 633
RelHolCal-2004, p. 96
SaintFestCh-1904, p. 256
(c)