TRINITY SUNDAY, the Sunday next after Whitsunday. A festival in honour of the Trinity had been celebrated locally at various dates before Pope John XXII. in 1334 ordered its general observance on the octave of Whitsunday. According to Gervase of Canterbury, it had been introduced into England by Thomas Becket, archbishop of Canterbury, in 1162. It has, however, never been reckoned among the great festivals of the Church. From Trinity Sunday onwards all Sundays until the close of the ecclesiastical year are reckoned as " after Trinity." In the Roman Church these Sundays are also reckoned as " after Pentecost." In the latter case they are described as dominicae trinitatis, not to be confused with dominicae post trinitalis; e.g. Dominica sexta post trinitatis is the same as Dominica seplima trinitalis.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)