The day and date complications together with the time give a full picture of the day.  It allows for the wearer to know know the time of day, day of the week, and date of the month.  The day and date complications are paired together, as knowing the day normally considered an addition to knowing the date.  Day of the week indicators are, similarly to the date, a pointer or a window.  Most of the time, the day and date are displayed in the same way (rather than having one pointer and one window).