This is something I’ve always been curious about. I love the empty windows right around Christmas, Thanksgiving, and July 4. The original article says that this data is about births in the US from 1973-1999. I would imagine that there are very few people born on September 11 in the last 12 years.
I’d also bet that this varies by occupation. For example, I’ve had friends who are teachers tell me that the best time to have a baby is 6 weeks before the end of the school year, so you can coast from maternity leave into summer vacation. Any teacher who gets pregnant in February or March is probably having a surprise.