subracting sundays and get the remaining days of the particular month

@ CD: you're using Convert.ToDateTime to get a DateTime object. That's a not a very good idea considering that some places use different format. Your code gives the wrong results for me because where I live we put the day before the month. So you may want to change it to this:
DateTime SDate = new DateTime(month: cal.Month, day: 1, year: cal.Year);

to achieve this task,

First you need to take First Date and last Date of selected Month

Using  DateTime.DaysInMonth  You can get no of Days in Month.

       DateTime SDate = Convert.ToDateTime(calDate.Month.ToString() + "/01/" + calDate.Year.ToString());
 
  int DaysInMonth = DateTime.DaysInMonth(cal.SelectedDate.Year, cal.SelectedDate.Month);
 
  DateTime EDate = SDate.AddDays(DaysInMonth - 1);

Now Find No Of Sunday 
 Int32 SunCnt = 0;
 
  while (SDate.DayOfWeek != DayOfWeek.Sunday)
  {
 
  SDate = SDate.AddDays(1);
 
  }
 
  SunCnt = 1;
 
  SDate = SDate.AddDays(7);
 
  while (SDate <= EDate)
  {
  SunCnt += 1;
  SDate = SDate.AddDays(7);
 
  }

Now Exclude sunday from Total no of Days and get your final result

       int FinalSalaryDay = DaysInMonth - SunCnt;
 


Full Source Code ::


 
 private string GetDaysInMonthExceptSunday(DateTime cal)
  {
 
  DateTime SDate = Convert.ToDateTime( cal.Month.ToString() + "/01/"+ cal.Year.ToString());
 
  int DaysInMonth = DateTime.DaysInMonth(cal.Year, cal.Month);
 
  DateTime EDate = SDate.AddDays(DaysInMonth - 1);
 
  Int32 SunCnt = 0;
 
  while (SDate.DayOfWeek != DayOfWeek.Sunday)
  {
 
  SDate = SDate.AddDays(1);
 
  }
 
  SunCnt = 1;
 
  SDate = SDate.AddDays(7);
 
  while (SDate <= EDate)
  {
  SunCnt += 1;
  SDate = SDate.AddDays(7);
 
  }
 
  int FinalSalaryDay = DaysInMonth - SunCnt;
 
 return  FinalSalaryDay.ToString();
  }

A method that returns the amount of days in a specific month of a specific year without counting the sundays?

You could skip the first week (incomplete or not, will include the first sunday) and divide the rest of the days by 7 to get the amount of sundays. and subtract that number from the total amount of days in the month (not to forget the first sunday so subtract 1).

int GetDaysExlSunDay(int year, int month)
{
    //create a data set to first day of that month of that year
    var date = new DateTime(year: year, month: month, day: 1);

    //get the amount of days in that month
    int daysInMonth = DateTime.DaysInMonth(date.Year, date.Month);

    //temporal variable to see how many days are in the first week
    int tmp = 0;

    //fast forward to first day after the first sunday
    do
    {
        date = date.AddDays(1);
        tmp++;
    }
    while (date.DayOfWeek != DayOfWeek.Monday);

    //subtract amount of sundays and not to forget the first sunday
    return daysInMonth - (daysInMonth - tmp) / 7 - 1;
}

Or a much easier to understand method (but perhaps a tiny bit slower). This method just keeps adding days and adding one to the result if it's not sunday. until the next month is reached.

int GetDaysExlSunDay(int year, int month)
{
    //create a data set to first day of that month of that year
    var date = new DateTime(year: year, month: month, day: 1);
    int res = 0;

    while (date.Month == month)
    {
        //if not sunday add one to res
        if (date.DayOfWeek != DayOfWeek.Sunday)
        {
            res++;
        }

        //go to next day
        date = date.AddDays(1);
    }

    return res;
}