I imagine there is some (probably thin) after-the-fact biblical interpretation that justifies scheduling it like that, and all the order of days and such is more closely related to the bible, so its just the placement that sticks out. But I think it's because after the bible was written, the authority for determining questions like that was passed to the church directly by Christ. As the one true church, they then could faithfully determine ambiguous things as would be willed by God. So they worked it out as God willed it more or less. But also I don't think the date itself is really considered special, nor is Christmas, it's the things you do on the days that lend it meaning. There is no ritual significance to the equinoxes (def a symbolic one though), no spell you have to cast under the correct celestial configuration to ensure anything, the ritual significance is the Mass and all of that jazz, and that's just the time they picked to do that stuff cause everyone must do it at the same time