St. Peter ad Vincula
Several antiquaries beginning with John Brady offered a back-construction to its being originally known as "Lamb-mass", under the undocumented supposition that tenants of the Cathedral of York, dedicated to St. Peter ad Vincula, of which this is the feast, would have been required to bring a live lamb to the church, or, with John Skinner, "because Lambs then grew out of season." This is a folk etymology, of which "OED" notes that it was "subsequently felt as if from ".