Encyclopædia Britannica, Ninth Edition/Trinity Sunday

TRINITY SUNDAY, which immediately follows Whit sunday, was in the older liturgies regarded merely as the "Octave" of Pentecost. The habit of keeping it as a distinct festival seems to have sprung up about the llth century. According to Gervase of Canterbury, it was Thomas Becket who introduced it into England in 1162. The universal observance of it was established by Pope John XXII. in 1334.