Linq query fails with "The specified type member 'Date' is not supported in LINQ to Entities."

Hi,
The following Linq query fails with "The specified type member 'Date' is not supported in LINQ to Entities."

viewModel.InvoiceLessons = from ss in db.StudentSemesters join l in db.Lessons on ss.Id equals l.StudentSemesterId join cd in db.CalendarDates on l.Date.Date equals cd.Date join t in db.Tariffs on l.TariffId equals t.Id where ss.Id == id select new PreliminaryInvoice {..... };
>>> join cd in db.CalendarDates on l.Date.Date equals cd.Date 
 cd.Date = 7/7/2013
is to join with all lessons on that day
eg. l.Date = 7/7/2013 15:00, 7/7/2013 15:30, 7/7/2013 16:00
regards, Guy