Link Errors - Please help me out

I was using factory pattern in my codes. But I got link errors when I
built my project, could anybody help me out please???? I have struggled
for this problem for one day :(

Any help would be greatly appreciated!

Errors Information:
error LNK2019: unresolved external symbol "protected: __thiscall
Event::Event(class std::basic_string<char,struct
std::char_traits<char>,class std::allocator<char> >,class
std::basic_string<char,struct std::char_traits<char>,class
std::allocator<char> >,class Date,class Date)"
(??0Event@@IAE@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@0VDate@@1@Z)
referenced in function "private: __thiscall
ConcreteEvent::ConcreteEvent(class std::basic_string<char,struct
std::char_traits<char>,class std::allocator<char> >,class
std::basic_string<char,struct std::char_traits<char>,class
std::allocator<char> >,class Date,class Date)"
(??0ConcreteEvent@@AAE@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@0VDate@@1@Z)

My codes:

/*.h file/
class Event {
     public:
        std::string description; // Description of Event
        std::string location; // Place where event is taking place
        const Date start; // Date and time when the event starts (inclusive)
        const Date end; // Date and time when the event ends (exclusive)

                static Event & make(Date,Date);

      private:
        static Event *_Ev;

    protected:
        // Event Constructor: newly created Event
        static Event *_Event;
        Event(std::string , std::string , Date , Date);
        // Destructor
        ~Event();
};

class ConcreteEvent:public Event{
    friend class Event;
    private:
        static ConcreteEvent *_concreteEvent;
        ConcreteEvent(std::string , std::string , Date, Date);
        ~ConcreteEvent();

    public:
        static ConcreteEvent * getInstance(std::string loc, std::string desc,
Date s, Date e) ;

};

/* .cpp file */

Event * Event::_Ev=NULL;
Event & Event::make(Date start,Date end)
{
    Event *e=0;
    if ((start<end) || (start==end))
    {
            e=ConcreteEvent::getInstance("","",start,end);
    }
    else
    {
        throw "The start date of an event cannot be later than the end
date!";
    }
    return *e;
}

ConcreteEvent::ConcreteEvent(std::string loc, std::string desc, Date s,
Date e):Event(loc,desc,s,e)
{}

ConcreteEvent::ConcreteEvent(Calendar c,Date s, Date e):Event(c,s,e)
{}

ConcreteEvent * ConcreteEvent::_concreteEvent=NULL;
ConcreteEvent * ConcreteEvent::getInstance(std::string loc, std::string
desc, Date start, Date end)
{
    if (_concreteEvent==0)
    {
        _concreteEvent=new ConcreteEvent(loc,desc,start,end);
    }
    return _concreteEvent;

}