[Table("DirCurrency")]
    public class DirCurrency
    {
        [Key]
        public int DirCurrencyID { get; set; }
        [Required]
        public int DirCurrencyCode { get; set; }
        [Required]
        public string DirCurrencyName { get; set; }
    }

    [Table("DirCurrencyHistory")]
    public class DirCurrencyHistory
    {
        [Key]
        public int DirCurrencyHistoryID { get; set; }
        [Required]
        public DateTime DirCurrencyHistoryDate { get; set; }
        [Required]
        public decimal? DirCurrencyRate { get; set; }
        [Required]
        public int? DirCurrencyMultiplicity { get; set; }
    }

var query =
    (
        from dirCurrencies in db.DirCurrencies

        // История
        join dirCurrencyHistories1 in db.DirCurrencyHistories on dirCurrencies.DirCurrencyID equals dirCurrencyHistories1.DirCurrencyID into dirCurrencyHistories2
        from dirCurrencyHistories in dirCurrencyHistories2.

        Where(
            y => y.DirCurrencyHistoryDate ==
                (
                    //!!! Вот из-за этого ошибка: "apply joins are not supported" !!!
                    from x in db.DirCurrencyHistories
                    where x.DirCurrencyID == dirCurrencies.DirCurrencyID && x.DirCurrencyHistoryDate <= _params.HistoryDate
                    select x.DirCurrencyHistoryDate
                ).Max()
        ).
        DefaultIfEmpty()

        /*
        //Так не правильно. Т.к. если нет данных в истории, то результат будут пустой
        where
            // История (выборка максимального из истории)
            dirCurrencyHistories.DirCurrencyHistoryDate ==
                (
                    from x in db.DirCurrencyHistories
                    where x.DirCurrencyID == dirCurrencies.DirCurrencyID && x.DirCurrencyHistoryDate <= _params.HistoryDate
                    select x.DirCurrencyHistoryDate
                ).Max()
        */

        select new
        {
            DirCurrencyID = dirCurrencies.DirCurrencyID,
            Del = dirCurrencies.Del,
            DirCurrencyCode = dirCurrencies.DirCurrencyCode,
            DirCurrencyGroupID = dirCurrencies.DirCurrencyGroupID,
            DirCurrencyName = dirCurrencies.DirCurrencyName,
            DirCurrencyNameShort = dirCurrencies.DirCurrencyNameShort,

            DirCurrencyRate = dirCurrencyHistories.DirCurrencyRate == null ? 0 : dirCurrencyHistories.DirCurrencyRate,
            DirCurrencyMultiplicity = dirCurrencyHistories.DirCurrencyMultiplicity == null ? 0 : dirCurrencyHistories.DirCurrencyMultiplicity
        }
    );