John Day is perhaps best known as the printer of John Foxe’s Actes and Monuments, usually referred to as Foxe’s Book of Martyrs. Other lucrative Elizabethan patents held by John Day, such as those for the Metrical Psalms, the ABC, and the Catechism, ensured considerable financial return and a revered, if not envied, status among Elizabethan printers. Only the Queen’s Printer, Christopher Barker, could compare with Day’s volume of output; each man printed in excess of 350 works. Scholarship has primarily focused upon Day’s Edwardian and Elizabethan activities. Day’s whereabouts and activities have been debated and it has been assumed that any information about Day during Mary’s reign has been lost. However, by examining new evidence, new light may be cast not only on Day’s Marian activities, but also on clandestine printing in Tudor England and on William Cecil.