For many people, this may not seem like a big deal. After all, with artificial lighting and whatnot, you can get chickens to lay during the winter no problem. However, we eschew the use of such lighting as we are allowing the chickens' bodies to go through their natural rhythm. We believe that this is part of obeying their design. After all, if they naturally stop laying, then they are probably not supposed to lay at that time.
This also helps us reduce feed costs. We do not supplement our chickens' diet with anything aside from what they can free range. And forcing them to lay during the winter would probably prevent us from doing so - we would have to feed them a bunch of extra food in order to give them the nutrients needed to produce eggs.
Anyway, that is why we are so surprised and blessed by this. As a chicken's natural egg laying is dependent upon the length of daylight, we were not expecting any until spring. But here we are, on the shortest day of the year, with a fresh egg. What a gift.