The calendar sections of illuminated manuscripts most often precede liturgical and devotional texts. In this context, they identify feast days pertinent to the PATRON and the region, using different colours to highlight important feasts, such as Christmas or the Annunciation (so-called red-letter days). Calendars vary in accordance with local USE, and the deaths and saints' feasts commemorated often supply valuable evidence of ORIGIN and PROVENANCE. Private, university, and official administrative texts also included calendars, which enabled the literate community to calculate dates. Calendars were often illuminated, the two most popular schemes being the labours of the months (see OCCUPATIONAL CALENDAR) and the zodiacal signs, both ultimately of classical origin but increasingly popular from the ninth century on. Calendars are often accompanied in religious books by devices for calculating movable feasts, such as Easter Tables. Medical and astronomical calendars appear in manuscripts relevant to those disciplines.
The Middle Ages inherited the Julian (Old Style) calendar introduced by Julius Caesar in 45 B.C. This contained a 365-day year, with an extra day every fourth year to reconcile the calendar with the solar year, calculated as 365 days and 6 hours. The year was divided into twelve months. Each month had named days: Kalends, Nones, and Ides, the unnamed days in between being reckoned backwards from the next Kalends, Nones, or Ides. Some months had dies Aegyptiacae ('Egyptian', or unlucky, days). Although commonly used, from EARLY CHRISTIAN times these Roman days competed with the ecclesiastical division of the year into weeks, each with seven named days, and with dating by reference to church feasts or occasions such as fairs and rent days.
The Roman civil year, beginning on January 1, continued to be used until the seventh century, when it was increasingly replaced by the Christian year, calculated from the year of Christ's birth, a system initially arising from the Dionysian Easter Table of c. 525 and popularized by the English scholar-theologian Bede during the eighth century. In this system Christmas, the Annunciation (March 25) or, less commonly, Easter marked the start of the year. Whatever the start of the year, the era began with the birth of Christ, the 'year of grace'.
Other calendrical styles were used in the Middle Ages as alternatives to or in association with the ecclesiastical 'year of grace'. Among these was the indiction, originally a civil reckoning that computed from 312 A.D. in fifteen-year cycles and was used for privileges and legal documents until relegated to notarial use in the late thirteenth century. Pontifical and regnal years also served calendrical purposes, relating a date to the person under whose jurisdiction the calendar was issued (for example, the second year of the reign of Henry III). Certain administrative offices had their own systems (the English Exchequer's financial year ran from Michaelmas, September 29, to Michaelmas). Spain, Portugal, and southwestern France used the Spanish Era calendar, beginning on January 1, 38 B.C., which survived in some areas until the fifteenth century.
The inclusion of devices such as the Golden Number, Epacts, Dominical letters, and Concurrents for the calculation of movable feasts (often added as tables) rendered a calendar perpetual or continually functional. The calculations were mostly concerned with establishing the relationship between the solar year and the phases of the moon so that the date of Easter could be determined.