For stackoverflow question at http://stackoverflow.com/questions/17204123/linq-retrieve-non-repeated-values#17204179 To run use linqpad and set it to C# program

solution.cs

void Main()
{
   List<Event_Day> Days = new List<Event_Day>();
   Days.Add(new Event_Day(420,120,1,new DateTime(2013,6,20)));
   Days.Add(new Event_Day(421,120,2,new DateTime(2013,6,21)));
   Days.Add(new Event_Day(422,120,3,new DateTime(2013,6,22)));
   
   List<Event_Session> Sessions = new List<Event_Session>();
   Sessions.Add(new Event_Session(170,120,420));
   Sessions.Add(new Event_Session(171,120,420));
   Sessions.Add(new Event_Session(172,120,420));
   Sessions.Add(new Event_Session(173,120,421));
   Sessions.Add(new Event_Session(174,120,421));
   Sessions.Add(new Event_Session(175,120,421));
   
  var unique_session = Sessions.Distinct(
    new GenComp<Event_Session>((a,b) => (a.Event_Id == b.Event_Id) && (a.Event_Day_Id == b.Event_Day_Id) , (a) => a.Event_Id.GetHashCode()+a.Event_Day_Id.GetHashCode()));
       
  var result = unique_session.Join(Days,
         s =>  new { Event_Id = s.Event_Id, Event_Day_Id = s.Event_Day_Id },
         d =>  new { Event_Id = d.Event_Id, Event_Day_Id = d.Event_Day_Id },
         (s, d) => new { Event_Day_Id = d.Event_Day_Id, DayNo = d.DayNo, DayDate = d.Day_Date });
         
  result.Dump();
   
}

public class GenComp<T> : IEqualityComparer<T> 
{
  public Func<T, T, bool> comp { get; private set; }
	public Func<T, int> hash { get; private set; }

	public GenComp(Func<T, T, bool> inComp, Func<T,int> inHash)
	{
		comp = inComp;
    hash = inHash;
	}

	public GenComp(Func<T, T, bool> inComp)
	{
		comp = inComp;
    hash = null;
	}

	public bool Equals(T x, T y)
	{
		return comp(x, y);
	}
  
  public int GetHashCode(T obj)  
  {  
    return hash == null ? obj.GetHashCode() : hash(obj);  
  }  
}

public class Event_Day
{
   public int Event_Day_Id { get; set; }
   public int Event_Id { get; set; }
   public int DayNo { get; set; }
   public DateTime Day_Date { get; set; }
   
   public Event_Day(int event_day_id, int event_id, int dayno, DateTime day_date)
   {
     Event_Day_Id = event_day_id;
     Event_Id = event_id;
     DayNo = dayno;
     Day_Date = day_date;
   }
}

public class Event_Session
{
   public int Event_Session_Id { get; set; }
   public int Event_Id { get; set; }
   public int Event_Day_Id { get; set; }
   
   public Event_Session(int event_session_id, int event_id, int event_day_id)
   {
     Event_Session_Id = event_session_id;
     Event_Id = event_id;
     Event_Day_Id = event_day_id;
   }
}