Note from Otto Ege: Breviaries were seldom owned by laymen. They were service books and contained the Psalter with the versicles, responses, collects and lections for Sundays, weekdays, and saints' days. Other texts could be included. A Breviary, therefore, was lengthy and usually bulky in format. Miniature copies like the one represented by this leaf are rare. The angular gothic script required a skilled calligrapher. It would be difficult for a modern engrosser to match, even with steel pens, the exactness and sharpness of these letters form with a quill by a 13th century scribe (note this is 15th century). Green was a decorative color added to the palette in the late 13th century in many scriptoria. The medieval formulae for making it from earth, flowers, berries, and metals are often elaborate and strange. This manuscript was written on fine uterine vellum, i.e. the skin of an unborn calf. It evidently had hard use, or may have been buried with its owner. 

Please note: Ege marks this as being from France and from the late 13th century. Scott Gwara (cited) marks it from c. 1425 from England.