error: invalid types ‘unsigned int[unsigned int]’ for array subscript

[thishas]

Why am I getting invalid type on

Project1.cpp:139:32: error: invalid types ‘unsigned int[unsigned int]’ for array subscript
os << Mon[month(date)-1];

void displayDate(const Date& date, ostream& os, DATE_STYLE ds)
{

 // if ds == MM_DD_YYYY)

  if (!wellFormed(date))
  {
     cout << "Date Error\n";
     exit(1);
  }
  else
    if (ds == MM_DD_YYYY)
    {
      if (numDigits(date) == 7)
        os << '0';
        os << month(date) << '/';
        if (nthDigit(date, 5) == 0)
           os << '0';
           os << day(date) << '/';
           unsigned y = year(date);
           unsigned len = 4 - numDigits(y);
           for (unsigned i = 0; i < len; ++i)
             os << '0';
             os << year(date) << endl;
     }
     else
       string Mon[12] = {"Jan ", "Feb ", "Mar ", "Apr ", "May ", "June ", "Jul ", "Aug ",
       "Sept ", "Oct ", "Nov ", "Dec " };
        unsigned Mon;
        os << Mon[month(date)-1];
        if (nthDigit(date, 5) == 0)
        os << '0';
        os << day(date) << ",";
        unsigned y = year(date);
        unsigned len = 4 - numDigits(y);
        for (unsigned i = 0; i < len; ++i)
        {
          os << '0';
          os << year(date) << endl;
        }
}