Museo Galileo
Virtual Museum
Dominical letter

In keeping with a medieval computation system, the first day of the year is assigned the letter from which the other letters of the alphabet follow until the first Sunday. From the letter indicating the Sunday (known, consequently, as the Dominical letter), we can determine the day in January when the first Sunday will fall. From that day, one can compute the other Sundays and the days of the week for the entire year. Using the Dominical letter in combination with the epact, one can determine the dates of Easter and all the other movable feasts.

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