Check this link Return date from previous week
Below expression returns Friday from previous week:
DayOfWeek WeekFriday = DayOfWeek.Friday;
int offsetDays = (int) WeekFriday - (int) DateTime.Now.DayOfWeek;
DateTime lastWeekFriday = DateTime.Now.AddDays(-7 + offsetDays);
or
DayOfWeek WeekFriday = DayOfWeek.Friday;
DateTime lastWeekFriday = DateTime.Now.AddDays(-7 + ((int) WeekFriday - (int) DateTime.Now.DayOfWeek));