Several factors might be at work here. One popular explanation is that it’s there is a “Christmas effect” resulting from either holiday merriment or being snowed in with nothing better to do. Both explanations have their weaknesses. For example, does your snow/holiday season start around October … or ever? Plus, I’d expect the major sexy holiday to be Valentine’s Day — and yet there is no uptick in births in early November. But this is, in fact, a testable hypothesis: What happens in other countries and climates?
Data from northern Europe show a strong seasonality trend, and this is a region that has a major winter holiday and plenty of snow. The problem is, conceptions there are lowest in winter — and they correlate inversely with the darkest days, not the holiday season. This pattern puts the major peak of births in spring; much of Europe has this spring peak, in contrast to the autumn peak in the US. But since 1970 or so, the pattern in Europe has been shifting, with autumn becoming the major peak. The reason isn’t clear. One paper suggests that partners who are separated might get together for the holidays. Another points out that decreasing spring conceptions could correspond with a declining trend in the European tradition of August vacations.