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.

Nine months after St.Patricks day, there's a blip on December 12. Interesting....