I always work on the assumption that my commitment is to the date I select and any other date within the window would require rescheduling. So lets say my choice was any date between the 1st and the 6th. I pick the 3rd as the day it looks like would work best for me. Some sites will allow me to reschedule myself earlier or later within the window, others require company or scheduler intervention. Absent a reschedule the shop must be done on the 3rd.
Usually I try for the first day of the window so if something comes up--vehicle problem, illness, mess up the shop or shop the wrong location--I can request a reschedule rather than needing to flat cancel the shop,