Meaning of cada tercer día by Anónimo

Anonymous, with all due respect, although I understand your logic of starting time at zero, we don't use that logic when we're practical in everyday life. If I'm going to give someone three seconds to drop something, I start counting from 1, not zero. When you tell him you'll see him soon you say: "In 1 minute I'll be with you." When you ask someone to give you a little more timeout you say, "Give me 1 second". In this way being practical we always start with 1 and calculate the smallest part of time as 1, never as 0 . 0 is nothing, so we can't count it pragmatically. If we say "every third day", we do not start counting the day on which we are as 0 but as 1 and in the common and practical use, we do not ignore it to the point of not telling it.