Calendars at the front of all Books of Hours told the date by citing the feast that was celebrated on that particular day. Today, when we speak of Christmas Eve, Christmas Day, and St. Valentine's Day, we know we are referring to December 24 and 25, and February 14. This is the medieval way of telling time.
The feasts listed in medieval Calendars are mostly commemoration of the day the saints were martyred (their “birthdays” into heaven). Other feasts commemorate important events in the lives of Christ and the Virgin. But no Calendars include the events of Christ's Passion (Resurrection, Ascension, or the Descent of the Holy Spirit): these were movable feasts whose dates depend upon that of Easter, the celebration of which changed from year to year. In a way, Calendars in Books of Hours are perpetual calendars since they can be used from one year to the next.
The majority of feasts are written in black (or dark brown) ink, whereas the more important feasts appear in red (hence, our term “red-letter day,” meaning a major event) or, sometimes, blue. Sometimes in deluxe manuscripts the most important feasts are written with gold leaf.
Along with the major feasts celebrated by the medieval Catholic Church as a whole, Calendars also include feasts of a more local interest. These are the ones that help determine the Calendar's “use,” the place where the manuscript was intended to be used. In addition to geographic uses, some Books of Hours were made for particular religious orders, such as Franciscan or Dominican.
The use of a Calendar can be helpful in determining where the Book of Hours was actually made. Paris and Dutch books of Hours were made locally, since the existence of productive workshops avoided the importation of manuscripts manufactured elsewhere. But many Horae with English or Spanish Calendars were manufactured in Flanders or France. The situation continued with printed Horae but their colophons or title pages often give, in addition to the manuscript's use, the name and city of the printer.
Letters (running from A through G) and Roman numbers (from I to XIX) appear to the left of the list of saints’ days: the Dominical Letters help finding Sundays and all the other days of the week throughout the year (each year this Sunday Letter changed, moving backward); the Golden Numbers indicate the appearances of new moons and full moons throughout the year (the latter by counting ahead fourteen days). This esoteric information was extremely important to the medieval Christian, since it helped determine the date of Easter, the Church's most important feast, in any given year.
Finally, many Calendars, especially those from the thirteenth to the mid-fifteenth century, include the ancient Roman calendrical system. Each month had but three fixed points: Kalends (always the first day of the month and from which we derive our term “calendar”), Ides (the middle of the month, either the thirteenth or fifteenth), and Nones (the ninth day before the Ides, counting inclusively; it fell on the fifth or seventh of the month). All the days in between were counted backward from these three fixed points.
Medieval time was Roman time. It followed the reformed but still imperfect system instituted by Julius Caesar (100-44 B.C.). Pope Gregory XIII (papacy, 1572-85) reformed the Julian calendar and, adding ten days (October 4 in 1582 was followed by October 15) and other fine tunings instituted in 1583 the Gregorian calendar we use today.