Our priest today, who happens to be French, taught us today that this is kind of a French Feast Day. It started in France in the 9th century, there-abouts, and was not approved by Rome until the 14th century, stating "every Sunday is a celebration of the Trinity." Our priest chuckled, then in the 14th century they elected a French pope - and he declared it to be a feast day for the whole Church.
Today we celebrate one of the foundations of Christianity - Three Divine Persons in One God. It is an extraordinary feast which we celebrate in Ordinal Time, the First Sunday After Pentecost.