We’ve already discussed desayuno (“breakfast”): breakfast is the break-fast, just like des- (“anti”) ayuno (“fast”)!
However, there’s an interesting addition to the story: dinner.
The English dinner originally comes from the French for breakfast, which is almost the same as the Spanish. Both are from Latin and meant the same: desjunare. Thus, we can see over time that the Latin for break-fast (dis- + ieiunus) became “breakfast” in both French and Spanish and then, via the French transformed into the English dinner while remaining with the same meaning in Spanish.
Therefore, we can see the d-(s)-n of desayuno map to the d‑n of dinner.
But all of this suggests a question: how did breakfast (the first meal of the day) turn into dinner (the last meal of the day)?
Easy: breakfast kept on getting later and later — until it was dinner!
At first it was eating in the morning: breaking the fast of the night. Then, over time, the big fast-breaking meal would happen around 2pm. Then eventually it turned into our 6pm dinnertime.
We see this vestige of the old usage in England, where dinner is sometimes used to refer to “lunch”–and the night-time meal that Americans call dinner is still sometimes called… supper.