A deeply internalized knowledge of the liturgical year and of the calendar was crucial. Just as we might casually refer to some event as having happened “the day before Thanksgiving” (on the Wednesday before the last Thursday in November), a notary in medieval Italy might date a will “feria quarta post festum sancte Catharine” (the Wednesday after the feast of St. Catherine of Alexandria). Both kinds of datings - for fixed and for moveable feasts - function as signposts for working through liturgical manuscripts.
The liturgical year, intricate and majestic in its great cycles, overlapping and interlocking, is a yearly reminder of the birth, passion, death, and resurrection of Christ; it represents the dynamic intersection of temporal and eternal. Given the weight of its message and the complexity of its calculation, it is small wonder that the liturgical calendar appears so baffling and is yet so fundamental.
The two largest cycles are those called the temporale and the sanctorale. The temporale (or Proper of Time) organizes the feasts related to the life of Christ including the moveable feasts of the year. Christmas, the birth of Christ, always occurs on 25 December and is therefore the primary fixed date in the temporale. Advent, however, is variable in that it covers the four Sundays before Christmas. The other major feast in the temporale is Easter, the resurrection of Christ, which is observed on the first Sunday after the full moon that happens on or immediately after the spring equinox, which is taken as occurring on 21 March. This requires concordance among four time-measuring systems: the phases of the moon, the course of the sun, the yearly cycle (the last two for the spring equinox), and the days of the week; it allows for Easter to fall on any Sunday from 23 March to 26 April (i.e., 22 March and 25 April before the Gregorian reform of the calendar). Understandably, all the feasts that are dependent upon Easter are also variable, whether preceding (Lent, Quinquagesima, Sexagesima, Septuagesima, working backwards) or following (Ascension, Pentecost, Trinity, Corpus Christi, working forwards). As a consequence, the summer Sundays between Pentecost/Trinity and Advent may also vary in number since their termini at both ends move; the same is true of the Sundays between Epiphany and Lent with a moveable terminus at the end.
The sanctorale (or Proper of Saints) is a simpler cycle. It gathers the fixed days of the feasts of the saints, and as such is familiar to most of us today: Valentine’s Day is on 14 February; St. Patrick is celebrated on 17 March. For a particularly important saint’s feast, a vigil is celebrated the evening before the actual feast day, and the feast is extended with commemorations throughout the following week, counting the days on both ends to form an octave.
Since the saint’s feast (on a fixed day) might coincide with a Sunday or any other moveable feast, a system for ranking the feasts evolved, in order to determine liturgical precedence and method of celebration: totum duplex, duplex, semiduplex, and so on form one system of classification. Calendars might instead specify the results of the classification, by the number of allotted readings at Matins (xii lectiones, ix lectiones, iii lectiones), by the level of the vestments (in cappis, in albis), or by the elaboration of ceremonial (cum candelis).
Hagiographic emphasis, especially if the saint is venerated only in one community, is of great help in localizing medieval liturgical manuscripts. The canonization date of a saint in the later Middle Ages can also be used to propose a terminus post quem for the production of the manuscript in which it is inscribed.