20 Percent: The diminished likelihood that you were born on Christmas, compared with an average day.

Perhaps largely because many cesarean births can be scheduled in advance, doctors and patients seem to avoid certain days — and choose certain others. So for 1969 to 1988 (birthday data are not available for succeeding years because of confidentiality restrictions), the birthrate was about 20 percent lower on weekends; 15 percent to 20 percent lower on New Year’s Day and July 4; about 5 percent lower on Leap Day (Feb. 29, every four years), April Fools’ Day and Halloween; and about 5 percent higher on Valentine’s Day.