From the artist's commentary:
What kind of day is 11/09?

11/09 is Good Orc Day! (Goroawase: "11/09" -> "i i o kyu" -> "ii oku" -> "good orc".) Enjoy it just in time for Nice Tits Day (11/08).
PS: Among R-18 boy's popularity, 11/09 hit the rank of 255th place! Bookmark, add tags, and comment, please, and thank you, everyone! ... I'm unexpectedly benefiting from this high ranking.