Seems that those holiday parties put couples in the mood because September 26th is the most popular day for giving birth. The date comes exactly 39 weeks and 2 days after Christmas Day.

Metro checked out the math and it looks like a whole season worth of mistletoe makes for a belated Christmas gift at the end of September and October. The top ten birthdates come between the end of September and the first two days of October.

Source: https://metro.co.uk/2019/09/26/september-26-popular-day-year-baby-nine-months-christmas-10807032/