Observations are assigned "plan windows" which are a subset of the observing opportunities dictated by the scientific and spacecraft constraints. For most observations, plan windows are 8 weeks in duration, but they can be shorter or longer depending on the visit's constraints. Barring unforseen circumstances, your visit will be executed within its plan window.
The detailed scheduling of observations is on a weekly basis and begins 11 days prior to the week of execution. Approximately 7 days before the week of execution the start and end time of a visit will be displayed on this page. In most cases the observations will be executed at this time, but it is possible that the timeline can be disrupted by instrument or spacecraft problems or target of opportunity observations.