C++: what did I do wrong?

#include <iostream>

enum month {jan,feb,mar,apr,jun,jul,aug,sep,oct,nov,dec};
enum weekday {sun,mon,tue,wed,thur,fri,sat};

int year;
int numberofdays(month);
void plusone7(weekday&);

int main()
{
    int number = 0;
    int suns = 0;
    //Some code
    std::cout << number << " days total." << std::endl;
    weekday dayofweek;
    for (int day = 1,dayofweek = mon;day<=number;day++,plusone7(dayofweek)) //"No matching function for call to plusone7"
    {
        if (dayofweek == sun)
        {
            suns++;
        }
    }
    std::cout << suns << " is the number of sundays" << std::endl;
}

//Defines a few functions

void plusone7(weekday& added)
{
    if (added == sat)
    {
        added = sun;
    }
    else
    {
        added++;
    }
}

    for (int day = 1,dayofweek = mon;day<=number;day++)
    {
        if (dayofweek == sun)
        {
            suns++;
        }
        plusone7(dayofweek);
    }