The way this and the last few events have been formatted:
A) Two of the world's bosses have a new drop only available during the event (in this case it's Festival Solveig (Pikachu costume on one side of the image and Wonder Woman costume on the other side of the image, and yes those are for different affection levels) and Festival Bonny (the one in that green and purple outfit (whatever that's supposed to be referencing))
B) The koban paid pachinko has the chance to drop the two girls from A plus three more event drops (in this case Festival Regine (tavern wench costume in the mobile picture, no idea what he costume in the web picture is supposed to be), Festival Titania (the one with the skin that looks like outer space), and Labor Day Girl (mobile image for her is obvious reference to that "We can do it!" poster from US history, web image might be going for the character Fix It Felix from the movie Wreck It Ralph?))
C) There's a special chain of daily missions that only appear during the event, 14 in total. Complete at least 12 of the 14 event dailies to earn the final event girl (in this event it's Geekette (pictured in both images in a GameBoy dress))