Origins
A schedule was once just a scrap of papyrus. The word descends from Late Latin schedula, a diminutive of scheda — 'a strip of papyrus' — which itself came from Greek σχέδη (schédē), a papyrus leaf. For centuries, the word referred to the physical object, not the plan.
In medieval legal usage, a schedule was a supplementary document — a small strip of parchment attached to a larger one, carrying additional details. Tax schedules preserve this sense: they are appendices to the main tax code, each on its own metaphorical slip of paper.
Greek Origins
The pronunciation war between British /ˈʃɛd.juːl/ and American /ˈskɛd.juːl/ is one of the most famous splits in English. The American form follows the original Greek more closely, where σχ was pronounced /sk/. The British form reflects French influence, which softened the opening cluster to /ʃ/.
Spanish cédula took a different path entirely. A cédula in Spain and Latin America is an identity document — your personal strip of paper proving who you are. The same root that organises your Monday morning also proves your citizenship across the Spanish-speaking world.