Modus numerandi Romanorum ab eo nostro differt. Ei homines utramque finem includunt.
The Romans count differently from us by counting both ends. We say "every second // every other"; they say "every third".
A&G §631d: "The point of departure was, by Roman custom, counted in the reckoning, the second day being three days before, etc."
I'm writing in Latin hoping for correction, and not because I'm confident in how I express myself. Latinè scribo ut ab omnibus corrigar, non quod confidenter me exprimam.