I think the letter was sent on the 25th of September. This is because for Lizzie's murder on the 29th he could have been planing to cut her ear lobes as referred to in the letter. However he could have been afraid that he could get caught, as the man who found her body, his horse was spooked. The following murder which was the day after, the body was very badly mutilated with the earlobes cut unlike last time. This is is possibly a reason why the two murders were so close together, so he could do what he originally planned to do. I also believe that the letter was real as it matches what happened with the murders.