Related trivia: Easter and Passover actually overlap fairly often - they are both approximately the first full moon after the equinox, though details differ due to the actual calendars being used. Ramadan, on the other hand, becomes earlier each year as the Islamic calendar is purely lunar, and thus has "years" that are shorter than a solar year, so the dates cycle through a solar calendar over the years. The overlap with Easter will occur every X years based on the calendar length.