Calendars are expected components of the missal and the breviary (but not of their correlating chant books, the gradual and the antiphoner). The calendar retains the beginning of the Roman year with 1 January, while the liturgical year, originally understood to begin at Christmas, eventually settled between the ninth and the eleventh centuries on the pre-Christmas period of preparation as its beginning, whether that period was taken to last four weeks or six (as was normal in Spain). Thus, the texts of missals and breviaries begin with Advent (in the temporale) and, usually, the feast of St. Andrew (30 November; in the sanctorale).
Another anomaly is the martyrology: not strictly speaking a liturgical book, it nevertheless may have a calendar at the beginning, although its text doesn’t necessarily begin at Advent as in other liturgical books. The calendar-like material that precedes each day’s list of saints in a martyrology gives the phase of the moon.
The main feature of the calendar in a liturgical book is that it is perpetual. Because of this, it cannot associate names of the days with the sequential list of days in each month, nor can it include the moveable feasts of the temporale. These vital questions –which day is Sunday? which day is Easter?– are addressed by means of the Golden Numbers and the Dominical Letters, which usually appear in the calendar in the two first columns on the left in the presentation of each month.
The Golden Numbers represent the 19-year solar cycle and its 235 lunations that bring the sun and the moon back into perfect reconciliation; the system allows one to predict the “Paschal Term,” which is to say the full moon that controls Easter (since Easter must occur after the first full moon that is on or after the spring equinox). To predict the actual date of the Sunday that is Easter, one then turns to the Dominical Letters. These are the seven letters, A-G, in the second column on the left, in which the letter A is often larger or more elaborate than the others; beginning with A on 1 January, the letters repeat in the same sequence for all 52 weeks + 1 day of the year. Because of this one extra day, 1 January moves ahead by one named day each year: in 2018, 1 January was a Monday, in 2019 it fell on a Tuesday. Thus 2018 was a G year (since its first Sunday fell on the 7th day after 1 January; all other Sundays of the year, including Easter Sunday, will continue that sequence), 2019 was an F year (since its first Sunday fell six days after 1 January), 2020 was an E year, and so on.
More than one inexperienced scholar has taken the decorative A on 1 January to represent a real Sunday and thus dated his manuscript to one of the several possible years when Sunday fell on the first of the year. Another pitfall for the unwary lies in the occasional presence of an entry for “Resurrectio Domini” on 27 March; the entry sometime reads, with more accuracy, “Resurrectio Domini vera,” meaning that in the year 33, the first Easter Sunday occurred on that date.