Query multiple tables (2)

I have 2 tables in one database: first has an information about apatrments, second show what apartments have already booked: has ApatrmentIds from first one, and 2 dates: arrive and departure.

I need to show users what apartments are available to book. It means that user inputs date and apartments dates of book don't match.

Model of first table:

        public class Apartment
        {

            public int ApartmentId { get; set;}

            [Required]
            public int Quantity { get; set; }

            [Required]
            public Classes Class { get; set; }

            [Required]
            public int Price { get; set; }

        }

        public enum Classes
        {
            Economy,
            Standart,
            Luxury
        }

Model of second table:

  public class Booked
    {
        public int BookedId { get; set;}

        public int ApartmentId { get; set; }

        public DateTime Start { get; set; }

        public DateTime End { get; set; }
    }

I tried to use linq, but i need to communicate between ApartmentId(key) of the first table and ApartmentId(not key) of the second.

How it would be better to do?