The Book of Hours was the popular personal religious manuscript of the medieval period, and the vast majority of the surviving examples begin with a devotional calendar of saints and feasts. The Corpus Kalendarium Database, or CoKL DB, is a database of these manuscripts and their calendars, recording the saints and feasts, and cross linking them to allow querying by metadata of the manuscript or calendar itself, or the presence and rank of a particular observance or group of observances. This paper presents an introduction to the underlying relational database, the user interface, and some of the ways that this data-driven system can present manuscripts which would be impossible with the physical objects.