Calendars were typically placed at the front of a Book of Hours and other service books like psalters, breviaries, and missals. They served to identify feast days and saints that were important in the region or diocese where the book was produced or for which it was intended. Unlike modern calendars that must be replaced annually, these calendars could be used year after year. The page for each month includes a number of columns. The column of letters a through g, called the Dominical Letters, could be used to find the dates on which Sundays fell throughout the year. Together with the left-most column of numbers, called the Golden Numbers, they were used to calculate the date of Easter, a lunar holiday. In the Chew Psalter, shown here, ordinary saints' days are written in black ink (now faded to brown), more important occasions are inscribed in red or blue, and the most important feasts recorded in gold. The common practice of using red ink for feast days gave rise to the phrase "red letter day".
Localization and Special Saints:
Sometimes it is possible to discover from the calendar of a Book of Hours the saints who were especially revered in the region in which the manuscript was produced, or saints to whom the owner had a special devotion. An example of such localization may be found in the Lawrence Hours, where St. Hubert is named twice - on September 8 as well as on his feast day, November 3. Hubert was the patron saint of Belgium, as well as the protector of archers, hunters, and dogs.