Mary Kelly lived at 13 Miller's Court in 1888. On 29 September, she couldn't find her rent. The next day, she was in the Ten Bells pub trying to get Bert to let her and her friend Mary Warner stay the night or to front her some shillings. In the original timeline, she died a year or so after that.
Events changed when Amy Pond went to tell Mary Warner that she would be the next victim. Mac'atyde (the Ripper) would instead abduct the woman Amy warned to run from him. When the Eleventh Doctor and Frederick Abberline organised a stakeout on 8 November, they weren't expecting the Ripper to change targets. At 10 am on 9 November, a child informed Abberline that there had been a murder in 13 Miller's Court. The Doctor felt the universe rewriting itself.