It could be plausible that the changes of weather and other conditions throughout the year might, at some level, influence our personality depending on when we are born. People participate in different activities depending on the time of year, and have different habits. For these reasons, the chemicals running through the mother would differ slightly or have different concentrations, and that would influence the baby in the womb. Perhaps their is an increase of "introverted" children that are born in Winter, and slightly more "extroverted" ones in Summer, because of the contrast in general behaviors people have in these months. So the month one is born could in theory affect who you are, if only slightly.
The issue with this of course is that, unlike Astrology, it could never be universal, because the weather and seasons differ from country to country. The culture would also have to be taken into account. And some places only have a dry or wet season, so the divisions aren't even uniform. But what I'm getting at is that Astrology's premise of the time of year affecting personality (if it is due to positions of the sky rather than weather etc) isn't completely nonsensical.