我是这样做的:

DateTime.Now.Date.AddDays(-(DateTime.Now.Date.DayOfWeek == 0 ? 7 : (int)DateTime.Now.Date.DayOfWeek) + 1)

这段代码所做的就是从给定的日期时间中减去天数。

如果星期几是0(星期日)，那么减去7，否则减去星期几。

然后在前一行的结果上加上1天，这将给出该日期的星期一。

这样你就可以在最后摆弄数字(1)来得到你想要的那一天。

private static DateTime GetDay(DateTime date, int daysAmount = 1)
{
    return date.Date.AddDays(-(date.Date.DayOfWeek == 0 ? 7 : (int)date.Date.DayOfWeek) + daysAmount);
}

如果你真的想使用DayOfWeek枚举，那么可以使用这样的东西…虽然我个人更喜欢上面的那个，因为我可以增加或减少任何数量的天数。

private static DateTime GetDayOfWeek(DateTime date, DayOfWeek dayOfWeek = DayOfWeek.Monday)
{
    return date.Date.AddDays(-(date.Date.DayOfWeek == 0 ? 7 : (int)date.Date.DayOfWeek) + (dayOfWeek == 0 ? 7 : (int)dayOfWeek));
}

var now = System.DateTime.Now;

var result = now.AddDays(-((now.DayOfWeek - System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek + 7) % 7)).Date;

下面的方法应该返回所需的DateTime。周日为true，周一为false:

private DateTime getStartOfWeek(bool useSunday)
{
    DateTime now = DateTime.Now;
    int dayOfWeek = (int)now.DayOfWeek;

    if(!useSunday)
        dayOfWeek--;

    if(dayOfWeek < 0)
    {// day of week is Sunday and we want to use Monday as the start of the week
    // Sunday is now the seventh day of the week
        dayOfWeek = 6;
    }

    return now.AddDays(-1 * (double)dayOfWeek);
}

从Compile This' answer开始，使用以下方法获取一周中任何一天的日期:

public static DateTime GetDayOfWeek(DateTime dateTime, DayOfWeek dayOfWeek)
{
   var monday = dateTime.Date.AddDays((7 + (dateTime.DayOfWeek - DayOfWeek.Monday) % 7) * -1);

   var diff = dayOfWeek - DayOfWeek.Monday;

   if (diff == -1)
   {
      diff = 6;
   }

   return monday.AddDays(diff);
}

稍微啰嗦一点，了解一下文化:

System.Globalization.CultureInfo ci = 
    System.Threading.Thread.CurrentThread.CurrentCulture;
DayOfWeek fdow = ci.DateTimeFormat.FirstDayOfWeek;
DayOfWeek today = DateTime.Now.DayOfWeek;
DateTime sow = DateTime.Now.AddDays(-(today - fdow)).Date;

这是一个正确的解决办法。无论一周的第一天是周一、周日还是其他日期，下面的代码都能正常工作。

public static class DateTimeExtension
{
  public static DateTime GetFirstDayOfThisWeek(this DateTime d)
  {
    CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
    var first = (int)ci.DateTimeFormat.FirstDayOfWeek;
    var current = (int)d.DayOfWeek;

    var result = first <= current ?
      d.AddDays(-1 * (current - first)) :
      d.AddDays(first - current - 7);

    return result;
  }
}

class Program
{
  static void Main()
  {
    System.Threading.Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("en-US");
    Console.WriteLine("Current culture set to en-US");
    RunTests();
    Console.WriteLine();
    System.Threading.Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("da-DK");
    Console.WriteLine("Current culture set to da-DK");
    RunTests();
    Console.ReadLine();
  }

  static void RunTests()
  {
    Console.WriteLine("Today {1}: {0}", DateTime.Today.Date.GetFirstDayOfThisWeek(), DateTime.Today.Date.ToString("yyyy-MM-dd"));
    Console.WriteLine("Saturday 2013-03-02: {0}", new DateTime(2013, 3, 2).GetFirstDayOfThisWeek());
    Console.WriteLine("Sunday 2013-03-03: {0}", new DateTime(2013, 3, 3).GetFirstDayOfThisWeek());
    Console.WriteLine("Monday 2013-03-04: {0}", new DateTime(2013, 3, 4).GetFirstDayOfThisWeek());
  }
}