Saint Bede on Easter

Saint Bede writes, about 725 AD, in De ratione temporum 15:

15.  The English Months

In olden time the English people — for it did not seem fitting to me that I should speak of other people's observance of the year and yet be silent about my own nation's — calculated their months according to the course of the moon.  Hence, after the manner of the Greeks and the Romans (the months) take their name from the Moon, for the Moon is called mona and the month monath.

The first month, which the Latins call January, is Giuli; February is called Solmonath; March Hrethmonath; April, Eosturmonath; May, Thrimilchi; June, Litha; July, also Litha; August, Weodmonath; September, Halegmonath; October, Winterfilleth; November, Blodmonath; December, Giuli, the same name by which January is called. …

Nor is it irrelevant if we take the time to translate the names of the other months. … Hrethmonath is named for their goddess Hretha, to whom they sacrificed at this time.  Eosturmonath has a name which is now translated "Paschal month", and which was once called after a goddess of theirs named Eostre, in whose honour feasts were celebrated in that month.  Now they designate that Paschal season by her name, calling the joys of the new rite by the time-honoured name of the old observance.  Thrimilchi was so called because in that month the cattle were milked three times a day…

— From The reckoning of time, tr. Faith Wallis, Liverpool University Press 1988, pp.53-54

A page from Bede's De Tempore RationemThe page at right is from at 12th century manuscript digitied at the British Library. From that article:

‘The Reckoning of Time’ deals with computus, the science of time-reckoning and how to calculate the date of Christian holy days such as Easter. Composing the text for his pupils at Wearmouth-Jarrow, Bede used similes to communicate abstract ideas. Explaining that that Earth is shaped like a sphere, Bede said that it is ‘not circular like a shield or spread out like a wheel, but resembles a ball being equally round in all directions’. Because it was so clearly written, with examples for teachers and pupils, De temporum ratione became one of Bede’s most popular works, remaining a core ‘school book’ in western Europe for centuries.

More at the British Library 

From the Wikipedia article on this work:

The Reckoning of Time (Latin: De temporum ratione) is an English era treatise written in Medieval Latin by the Northumbrian monk Bede in 725. The treatise includes an introduction to the traditional ancient and medieval view of the cosmos, including an explanation of how the Earth influenced the changing length of daylight, of how the seasonal motion of the Sun and Moon influenced the changing appearance of the new moon at evening twilight, and a quantitative relation between the changes of the tides at a given place and the daily motion of the Moon.

The Reckoning of Time describes the principal ancient calendars, including those of the Hebrews, the Egyptians, the Romans, the Greeks, and the English. The focus of De temporum ratione was calculation of the date of Easter, for which Bede described the method developed by Dionysius Exiguus. De temporum ratione also gave instructions for calculating the date of the Easter full moon, for calculating the motion of the Sun and Moon through the zodiac, and for many other calculations related to the calendar.

Bede based his reasoning for the dates on the Hebrew Bible. The functions of the universe and its purpose are generally referred to a scriptural foundation. According to the introduction by Faith Wallis in the 1999 English translated edition of The Reckoning of Time, Bede aimed to write a Christian work that integrated the astronomical understanding of computing with a theological context of history. The book is also regarded by Bede to be a sequel to his works The Nature of Things and On Time.

Read more in The Reckoning of Time at Wikipedia

James Grout, in his Encyclopædia Romana, discusses Saint Bede's account of the development of the Paschal Calendar in Britain.