在“下周二”可能会有各种各样的内容，但是这段代码会给你“下个星期二发生，或者今天如果已经是星期二”：

DateTime today = DateTime.Today;
// The (... + 7) % 7 ensures we end up with a value in the range [0, 6]
int daysUntilTuesday = ((int) DayOfWeek.Tuesday - (int) today.DayOfWeek + 7) % 7;
DateTime nextTuesday = today.AddDays(daysUntilTuesday);

如果你想给“一周的时间”，如果它已经是星期二了，你可以使用：

// This finds the next Monday (or today if it's Monday) and then adds a day... so the
// result is in the range [1-7]
int daysUntilTuesday = (((int) DayOfWeek.Monday - (int) today.DayOfWeek + 7) % 7) + 1;

...或者你可以使用原来的公式，但从明天开始：

DateTime tomorrow = DateTime.Today.AddDays(1);
// The (... + 7) % 7 ensures we end up with a value in the range [0, 6]
int daysUntilTuesday = ((int) DayOfWeek.Tuesday - (int) tomorrow.DayOfWeek + 7) % 7;
DateTime nextTuesday = tomorrow.AddDays(daysUntilTuesday);

编辑：只是为了让这个更好：

public static DateTime GetNextWeekday(DateTime start, DayOfWeek day)
{
    // The (... + 7) % 7 ensures we end up with a value in the range [0, 6]
    int daysToAdd = ((int) day - (int) start.DayOfWeek + 7) % 7;
    return start.AddDays(daysToAdd);
}

因此，要获得“今天或未来6天”的值：

DateTime nextTuesday = GetNextWeekday(DateTime.Today, DayOfWeek.Tuesday);

要获得“下个星期二不包括今天”的值：

DateTime nextTuesday = GetNextWeekday(DateTime.Today.AddDays(1), DayOfWeek.Tuesday);

这应该能起作用：

static DateTime GetNextWeekday(DayOfWeek day)
{
    DateTime result = DateTime.Now.AddDays(1);
    while( result.DayOfWeek != day )
        result = result.AddDays(1);
    return result;
}