The algorithm to calculate the date of Easter every year. The word is the etymological root of 'computer' and 'computing' — originally people computed the computus. Specification evolved from 'Sunday after the first night of Passover' (required a rabbi in every village) to the 325 CE rule 'first Sunday after the first full moon on or after the vernal equinox', with the vernal equinox hardcoded to March 21st. Bede (725 CE) used the Metonic cycle (235 synodic months = 19 tropical years) to hand-compute the next 532 years of Easter dates. Still an open research problem — newer algorithms were published as recently as 1961. Framed in the talk as the original big data problem.