Someone innocently created a chart so that people could determine how common their birthday is (click here to see that chart). Then someone came along and thought, "huh, what if I shift all of those dates by 9 months?" Thus, through the magic of the internet, there is now a chart depicting the most common dates for people to make babies.
Click here for our article about the seasons when people have the most sex (not necessarily for the purposes of procreation).
image source: ilovecharts.tumblr.com