I agree with JF who suggested that this chart was mind-boggling. The chart accompanied a somewhat diffuse NYT article postulating that tax break or shifting medical practice or less apprehension about tired nurses or added labor-inducing stress from visiting relatives may have something to do with more babies being born in December, particularly at month's end.
This chart presumably shows the "spike" in December births, or more precisely, the shift of January births into December. The trouble with it is its lack of comparability. We need to compare the 2002-3 trend to some prior year to see the shift.
Even then, we would have seen only one data point. So it would have been better to plot multiple years.
Finally, after reading the article, I cannot discern the importance of Monday and Friday. The yellow-pink coloring has not improved my comprehension of the data; it leaves me with more questions than before.
Reference: "To-Do Lists: Wrap Gifts, Have Baby", New York Times, Dec 20 2006.
PS. Please now visit Jon's response. Kudos for digging out the historical data series and a stellar analysis!