Easter is the most important event in the Christian year, and the calculation of its date was paramount. The Computus served as a guide for this purpose.
Computus in its simplest definition is the art of ascertaining time by the course of the sun and the moon. This art could be and was a theoretical science, such as that explored by Johannes of Sacrobosco in his De sphera -a science based on arithmetical calculations and astronomical measurements derived from use of the astrolabe or, increasingly by the end of the 13th century, the solar quadrant. Computus is understood mainly, however, as the practical application of these calculations. To reckon time in the broadest sense and to determine the date of Easter became one and the same effort. And for most people, understanding the problem of correct alignment of solar, lunar, yearly and weekly cycles to arrive at the date of Easter was simply reduced to a question of “when?” rather than “why?”. The result was a profusion of calculation formulae, charts and memory devices.
Accompanying these handy mechanisms for determining the date of Easter were many other bits of calendrical information that faith, prejudice, and experience leveled to the same degree of acceptance and necessity: the lucky and the unlucky days for travel or for eating goose; the prognostications of rain or wind; the times for bloodletting; the signs of the zodiac; the phases of the moon; the number of hours of sunshine in a given day; the feasts of the saints; the Sundays in a perpetual calendar.