第一:不要编写自己的函数，如果已经有标准化的函数可以处理相同的问题。 要点是，您可能很容易犯错误（我已经在weekday()函数的第一行看到了一个错误，就像现在一样），而标准化函数的实现已经经过了彻底的测试，您可以确信它们提供了您期望得到的结果。

尽管如此，下面是一种可能的方法，使用std::localtime和std::mktime:

#include <ctime>
#include <iostream>

int main()
{
  std::tm time_in = { 0, 0, 0, // second, minute, hour
      9, 10, 2016 - 1900 }; // 1-based day, 0-based month, year since 1900

  std::time_t time_temp = std::mktime(&time_in);

  //Note: Return value of localtime is not threadsafe, because it might be
  // (and will be) reused in subsequent calls to std::localtime!
  const std::tm * time_out = std::localtime(&time_temp);

  //Sunday == 0, Monday == 1, and so on ...
  std::cout << "Today is this day of the week: " << time_out->tm_wday << "\n";
  std::cout << "(Sunday is 0, Monday is 1, and so on...)\n";

  return 0;
}

您可以使用Boost C++库中的公历日期系统来查找给定日期的星期几。 这里有一个简单的例子:

#include <boost/date_time.hpp>
#include <string>
#include <iostream>

const static std::string daysOfWeek[] = {
    "Sunday",
    "Monday",
    "Tuesday",
    "Wednesday",
    "Thursday",
    "Friday",
    "Saturday"
};

int getDayOfWeekIndex(int day, int month, int year) {
    boost::gregorian::date d(year, month, day);
    return d.day_of_week();
}

int main()
{
    const int index = getDayOfWeekIndex(30, 07, 2018);
    std::cout << daysOfWeek[index] << '\n';
}

此代码打印monday。

你对闰年的理解是不正确的:

闰年是每4年一个，除非它能被100整除，但即使这样，如果它能被400整除，它仍然是闰年。

关于如何计算“日数”(dn)的清晰简明的说明可以在这里找到。

一旦你有了日期(dn)，只需执行模数7。 结果将是星期几（dow）。

下面是一个示例实现（不检查date是否为有效输入）:

#include <iostream>
#include <iomanip>

typedef unsigned long ul;
typedef unsigned int ui;

// ----------------------------------------------------------------------
// Given the year, month and day, return the day number.
// (see: https://alcor.concordia.ca/~gpkatch/gdate-method.html)
// ----------------------------------------------------------------------
ul CalcDayNumFromDate(ui y, ui m, ui d)
{
  m = (m + 9) % 12;
  y -= m / 10;
  ul dn = 365*y + y/4 - y/100 + y/400 + (m*306 + 5)/10 + (d - 1);

  return dn;
}

// ----------------------------------------------------------------------
// Given year, month, day, return the day of week (string).
// ----------------------------------------------------------------------
std::string CalcDayOfWeek(int y, ul m, ul d)
{
  std::string day[] = {
    "Wednesday",
    "Thursday",
    "Friday",
    "Saturday",
    "Sunday",
    "Monday",
    "Tuesday"
  };

  ul dn = CalcDayNumFromDate(y, m, d);

  return day[dn % 7];
}

// ----------------------------------------------------------------------
// Program entry point.
// ----------------------------------------------------------------------
int main(int argc, char **argv)
{
  ui y = 2017, m = 8, d = 29; // 29th August, 2017.
  std::string dow = CalcDayOfWeek(y, m, d);

  std::cout << std::setfill('0') << std::setw(4) << y << "/";
  std::cout << std::setfill('0') << std::setw(2) << m << "/";
  std::cout << std::setfill('0') << std::setw(2) << d << ": ";
  std::cout << dow << std::endl;

  return 0;
}