You will recall that the locative case was one of the Proto-Indo-European cases that melded with the ablative in Latin. The job of the locative case was to show location. Given the Roman time/space equation, location on a calendar makes just as much sense as location on a map. The Ablative of Time When or Within Which does just that. It specifies a time or a time range. As with the other time expressions, no preposition is used in the Latin.