包含不同类型参数的构造函数

谢谢两位.
不过我还是出了一个错:
error C2512: 'Day' : no appropriate default constructor available
怎么改啊?
#include <iostream>
using namespace std;
class Day
{
private:
int year,month,day;
public:
Day(int y,int m,int d)
{ year=y; month=m; day=d ; }
Day (Day &one)
{ year =one.year; month=one.month; day=one.day; }
void show()
{ cout<<year<<' '<<month<<' '<<day<<endl;}

};
class person
{
private:
int num,IDnum;
char sex;
Day birthday;
public:
person(int n ,int IDn)
{ num=n; IDnum=IDn; }

person( char s)
{ sex=s;}
person( Day &bir)
{ birthday= bir;}
void show ()
{ cout<<num<<' '<<IDnum<<' '<<sex<<endl; }

};
int main()
{

return 0;
}