There are four weekends in the month though, and Billy's photo session happens on the second one, which isn't the same as the Perry's beach house trip, which coincides with Alison's trip, if am not mistaken.
edit: i'd guess the schedule might go like, first week: Perry's beach house, second week: Billy's session / MMA tournament preliminaries? Although maybe EK will want to move the beach trip to 3rd week and one chapter later, to make it easier for themselves and avoid scene overload. So instead it could be MMA preliminaries in the first week and book contest in the second week for Ian.