Meaning of cada tercer día by furoya

Let's start by clarifying that it's not a locution. But it still occurs to me that someone might come to a dictionary to clear up their doubts. Which evidently do not come from the motto at the top of the page but from the entry provided by Cesar Orozco. The first thing that surprises me is that he says that he is an English teacher, because a doubt like this is typical of a programming class; Whoever interprets it correctly has a good chance of passing the course. And it's been so many years since they've been resurrecting it here that I decided to do my brief and humble work. . . No, my extensive and pedantic contribution. Let's start by separating two concepts: "every 3 [something]" is not the same as "every third [something]". The first case is counted with numerals , and the second with ordinals . In numbers, the beginning is counted from 0 ( zero ) and we take it for granted that there is a sequence; In order it is counted from the first of the elements and by definition we know that there is a sequence . I mean that in a collection the first element is the one numbered 0; the second as 1; the third as 2 . . . And then, taking the original example, "every third day" from a Monday you count "Monday, Tuesday, Wednesday", and in "every 3 days" you start from the first shot of the medicine as 0 (because you can't count anything backwards) and "1=Tuesday, 2=Wednesday, 3=Thursday". Of course this would be if we use logic and not tradition or custom, because someone may have been taught differently and will end up taking their medication any day, that's why the "every other day" (or "day in between", as they call it in each place) was invented, which leaves no doubt: it is "one day, Yes; another, no." For longer periods, it is customary to use the hours since 12; 24; 48; 72 hours leave no doubt and we understand that the first shot is moment 0.