Considering Frank and Helen came from our world, too, I've always thought (always since I began thinking about it) it had existed side by side from the beginning, or evolved side by side from the beginning, anyway. I think the natural-world names must be more commonly used, in Narnia at least, because they would be more obvious to the Beasts and other beings; who's August again?
![Wink ;-)](./images/smilies/icon_e_wink.gif)
It's also quite possible that Telmarines, having also come from Spare Oom and not being in contact with nature and Beasts and whatnot, would use the Latin/English calendar more, or even exclusively.