// This function will return the next saturday for a datetime
DateTime NextSaturday(DateTime now)
{
   while (now.DayOfWeek != DayOfWeek.Saturday)
      now = now.AddDays(1);
   return now;
}

更新

两年后，我想改变这个答案。
这些天来，我永远不会为class创建“实用函数”。我现在总是“延伸”课堂。签名现在应该是DateTime.Next(DayOfWeek)。有关扩展的更多详细信息，请参见http://msdn.microsoft.com/en-us/library/vstudio/bb383977.aspx。
逻辑也是错误的。如果now是星期六，那么它将始终返回当前日期。我想大多数打电话的人都希望它返回now + 7天。第一个变化是：

DateTime NextSaturday(DateTime now)
{
   do {
      now = now.AddDays(1);
   } while (now.DayOfWeek != DayOfWeek.Saturday)

   return now;
 }

然后将函数更改为适用于一周中的任何一天：

DateTime Next(DateTime now, DayOfWeek nextDay)
{
   do {
      now = now.AddDays(1);
   } while (now.DayOfWeek != nextDay)

   return now;
 }

现在“扩展”DateTime类以支持Next（DayOfWeek）

namespace DateTime.Extensions
 {
   public static class DateTimeExtensions
   {
     public static DateTime Next(this DateTime now, DayOfWeek nextDay)
     {
        do {
          now = now.AddDays(1);
        } while (now.DayOfWeek != nextDay)

        return now;
      }
   }
 }

DateTime add = DateTime.Now; //From popup box
int add = (((int)selected.DayOfWeek) + 1;
if(add != 7) {
    selected = selected.AddDays(add);
}

public DateTime DayInWeek(DateTime self, DayOfWeek dayOfWeek) 
{
        DateTime lRet = self;    
        int diff = (self.DayOfWeek - dayOfWeek) % 7;
        lRet = self.AddDays(-1 * diff).Date;
        return lRet; 
}

假设星期日是一周的第一天