Same here. I wondered how that was even possible before discovering my Lemmy client had cropped out the legend. Not a fan of this coloring scheme, especially having white as the median value.
Possibly chosen? My brother was born on the 1st of the month because my mom had a planned C-section and they gave her a window and let her pick the date. She picked something easy to remember. Although this may be less common because it's no longer mandatory to have a c-section just because your previous birth was via c-section, afaik.
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