Heat map of the birth ratio of each day to the average in the USA for people born from 1994 to 1999 according to the Center for Disease Control and 2000 to 2014 according to the Social Security Administration (top), and in Wales and England for people born from 1995 to 2014 according to the Office for National Statistics (bottom), also aggregated by month and day of month with 29 February normalised, and minima and maxima in bold by CMG Lee.
* Most common birthdays are in September.
† Least common birthdays fall around Christmas, New Year's and April Fools' Days, plus in the USA, Independence Day, Hallowe'en and Thanksgiving.
‡ The 13th of each month has fewer birthdays than adjacent days.
♥ Valentine's Day is overrepresented in the USA.

There is a weak weekly cycle due to the small number of years taken, but overall, England and Wales have a more uniform distribution. Data is from http://github.com/fivethirtyeight/data/tree/master/births and http://ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/livebirths/articles/howpopularisyourbirthday/2015-12-18 . In the SVG file, hover over a date to highlight it on both tables.
Own work
Cmglee