A friend of mine just posted a link on Facebook to this interactive "heat map" that shows how common different birthdays are. You should check it out -- it's pretty interesting, and the interface is cool too. (Just hover over a birthday to see its rank.)
After Leap Day, the least common birthday is Christmas Day, followed by New Year's Day. It's a pretty quick visualization, also noted on the blog, that July, August and September are the most common birth dates, so folks do tend to conceive during the winter holidays.
But the December data particularly stands out to me because my son Aaron's due date was Dec. 25, and I wanted him to be born basically any day around then, but not on Christmas. But then once we passed his due date, I wanted him to be born before the end of the year. (It wasn't about tax deductions, necessarily, but about insurance complications and that my mom, who was out here to help me for the birth since my husband was deployed, was going to have to leave pretty soon.)
The inference drawn on the linked blog are that doctors don't like to work holidays (July 4 and Thanksgiving week dates are also pretty low on the list). I'm sure that plays into it some, but I kind of feel like, as an expectant mother with Christmas due date, I was willing myself not to go into labor on the 25th. It was the first Christmas my older son really understood what was going on, and I didn't want to have to rush off to the hospital in the middle of the festivities. I'm sure I'm not alone! Who knows if my wishes or my will had any sort of an effect, but maybe? (As for whether the non-holiday birth dates are due to an escalating C-section rate, this data is for births between 1973 and 1999.)
And incidentally, my birthday is No. 200.