Answer:
Wednesday
According to the Census Bureau (citing the National Center for Health Statistics) , Wednesday is now the most common day of the week to deliver, with an average of 13,482 births taking place on Wednesdays (albeit during 2006).
This is the first time since at least 1990 that a day other than Tuesday had this distinction.
You can read the report at http://www.cdc.gov/nchs/data/nvsr/nvsr57/nvsr57_07.pdf