Why the Nativity of St. John on the 24th of June

Has it ever occurred to you that that's a bit odd? Annunciation: March 25. Nativity of Our Lord: December 25. Obviously the Nativity of St. the Baptist was set six months prior to our Lord's given the words of Archangel Gabriel in Luke 1:36. Why does the Nativity of St. John end up a day early? It goes back to the old Roman way of counting time, which counted the days BEFORE the month change (not after). Thus, Annunciation falls seven days before the Kalends of April; Christmas falls seven days before the Kalends of January; and the Nativity of St. John, lo and behold, falls seven days before the Kalends of July. So, since June has but 30 days, keeping to the seven day pattern would put you at June 24th. There, now that doesn't have to keep you up at night wondering anymore.