4 years ago I made a similar mistake (booked for the next day instead of the same day) and thought that I clicked in the flexible rate. Got the confirmation for the next day and the money charged instantly. Contacted Accor through Flyertalk and while they tried to help, was told nothing could have been done. Tried to book again as it was an emergency, but the hotel was full and could not find any other Accor, Hilton, or SPG property with availability. Fun times.