peak just before and after Christmas would indicate that it's day of birth, since hospitals work with reduced capacity on these days and induced births or c-sections will not happen then unless it's an emergency.
why so many births are on 14.02. is a mystery to me.
Best guess seems to be choice of day to induce labor within a given window. A lot of people, if given a window of Feb 7th through 21st or something, will choose the 14th for Valentine's Day