Niigata to Otaru ferry bookings

Hi, I'm trying to book a ferry from Niigata to Otaru for September 2025, via Direct Ferries. But there are no booking slots beyond June. Is this because it's a seasonal ferry, or because the September dates havent been released yet? If so, when will they come out? Thank you

Reservations are possible up to 3 months in advance. That is why there are no open dates for September. For example, you can start make reservations for September 3 from June 3 at 9am (Japan Time).