This side quest can be done during chapters 2, 3, 4, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, or 18.
You need to visit Sallie when she is at home, which is usually after midday. You can tell if Sallie is home by checking her house on the map, her icon will be there. You need to talk to her twice to activate the Where's the Dress! side quest.
Once the quest is activated go upstairs into Anna's room, and examine the photo of Anna in the dress. Now that York know what the dress looks like, go downstairs into the other bedroom to the rack of dresses to find the dress. Once you have the dress, go and take it to Sallie to complete the side quest.