This is the Computus, literally, “computation.”
It was created by Byrhtferth who was a priest and monk. He lived at Ramsey Abbey in Anglo-Saxon England. His work had deep impact on the intellectual life of the time and he wrote many computistic, hagiographic, and historical works.
The tables were based upon Abbo’s Computus. He studied under Abbo of Fleury and expanded upon his work to show a series of diagrams used for determining lunar cycles, days of the week, and divination diagrams based on numerical values assigned to the letters.
Byrhtferth’s digram is a visual meditation on the cosmic and religious resonances of computus, its subject the harmony of the twelve months and foursomes of science at the time eg. four ages of man, directions, humours, essences, winds, gospels, etc.