Why might a director schedule a run-through during the middle weeks of the rehearsal period?

A director will usually schedule a run-through during the 4th or 5th week of a rehearsal period. They will actually do the entire play from beginning to the end. This is done to make sure everyone knows their lines and spots. It is also a great way for a director to know if there are cast changes needed or a script change. This also shows the lights on the actors for the first time, the costumes, etc. During the run-through, the play is not stopped except for the scheduled intermissions. This also helps to show that the actors are ready to their jobs.