If you mean when the birthdays are, I believe that's good ol' internet information-hoarding at work. Each villager lets you know what their birthday is as the date draws near, so collecting the info on EVERYONE is just a matter of a lot of people contributing a date or ten each.I have no idea how people know that, but Nookipedia says so, so whatever.
*She^
I share mine with Fuchsia. Never had her, but she seems okay.