计算出从某一开始时间到某一截止时间中的周五与周六的天数！！！！

            int day = 0;
            DateTime beginDay = this.dateTimePicker1.Value;
            DateTime endDay = this.dateTimePicker2.Value;
     

            int n = 0;
            for (DateTime dt = beginDay; dt <= endDay; dt = dt.AddDays(1))
            {
                if (dt.DayOfWeek == DayOfWeek.Saturday || dt.DayOfWeek == DayOfWeek.Sunday)
                    n++;
                else
                    day++;
            }

            MessageBox.Show("当前有：" + n.ToString() + "个周未\r\n" + "合计有：" + day.ToString() + "平常日");

private void button1_Click(object sender, System.EventArgs e)
  {
   int day=0;
   DateTime beginDay =DateTime.Parse(this.textBox1.Text);
   DateTime endDay =DateTime.Parse(this.textBox2.Text);
   System.TimeSpan Days;
   Days=endDay-beginDay;
   day=(int)Days.TotalDays;   

   int y=0; //从开始时间算，计算出第一个周六。
   int x=0; //从开始时间算，计算出第一个周五。
   switch(DateTime.Parse(this.textBox1.Text).DayOfWeek)
   {
    case DayOfWeek.Monday: //周1
     y=-5;
     x=-4;
     break;
    case DayOfWeek.Tuesday: //周2
     y=-4;
     x=-3;
     break;    
    case DayOfWeek.Wednesday: //周3
     y=-3;
     x=-2;
     break;
    case DayOfWeek.Thursday: //周4
     y=-2;
     x=-1;
     break;
    case DayOfWeek.Friday: //周5
     y=-1;
     x=0;
     break;
    case DayOfWeek.Saturday:  //周6
     y=0;
     x=-6;
     break;
    case DayOfWeek.Sunday: //周7
     y=-6;
     x=-5;
     break;   
   }
   int d= (day + y) / 7;
   if((day + y) >=0)
   {
    d++;
   }

   int u= (day + x) / 7;
   if((day + x) >=0)
   {
    u++;
   }

   MessageBox.Show("当前有："+d.ToString() + "个周六\r\n" +  "当前有："+u.ToString() + "个周五\r\n"  +  "合计有："+ (d + u) + "天");
  }