Сортировать время и дату

Первое, что на ум приходит - разделить список на два сегмента: текущий день и все остальное.

int day = dateList.Select(day => day.DEN).Max();
IEnumerable<...> first = dateList.Where(day => day.DEN.Equals(day));
IEnumerable<...> other = dateList.Where(day => !day.DEN.Equals(day));

Потом сортировать их по отдельности как нужно и обратно объединить:

first = first.OrderByDescending(hours => hours.CAS);
other = other.OrderByDescending(day => day.DEN).ThenBy(hours => hours.CAS);
IEnumerable<...> sorted = first.Concat(other);

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Пример работы (Консоль .NET 6):

List<dateClass> dateList = new List<dateClass>()
{
  new dateClass(){ DAY = 03, HOUR = 18 },
  new dateClass(){ DAY = 04, HOUR = 18 },
  new dateClass(){ DAY = 04, HOUR = 16 },
  new dateClass(){ DAY = 03, HOUR = 17 },
  new dateClass(){ DAY = 03, HOUR = 16 },
  new dateClass(){ DAY = 04, HOUR = 17 }
};

int day = dateList.Select(x => x.DAY).Max();
IEnumerable<dateClass> first = dateList.Where(x => x.DAY.Equals(day));
IEnumerable<dateClass> other = dateList.Where(x => !x.DAY.Equals(day));
first = first.OrderByDescending(x => x.HOUR);
other = other.OrderByDescending(x => x.DAY).ThenBy(x => x.HOUR);
IEnumerable<dateClass> sorted = first.Concat(other);
Console.WriteLine(string.Join(Environment.NewLine, sorted.Select(x => x.DAY.ToString() + ": " + x.HOUR.ToString())));

class dateClass
{
  public int DAY { get; set; }
  public int HOUR { get; set; }
}

Выводит:

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Еще как вариант добавить столбцы нумерации из базы SQL. Например разные сортировки ROW_NUMBER() от DENSE_RANK() по дате:

SELECT
  [t],
  CASE
    WHEN [t] = 1
    THEN ROW_NUMBER() OVER (PARTITION BY [b] ORDER BY [a] DESC)
    ELSE ROW_NUMBER() OVER (PARTITION BY [b] ORDER BY [a])
  END AS [N],
  [b],
  [a]
FROM
  (
    SELECT
      DENSE_RANK() OVER (ORDER BY [b] DESC) AS [t],
      [b],
      [a]
    FROM
      (
        SELECT
          CONVERT([date], [a]) AS [b],
          [a]
        FROM
          (
            SELECT CONVERT([datetime], '2023-04-05 18:25:43', 120) AS [a]
            UNION ALL
            SELECT CONVERT([datetime], '2023-04-07 18:04:43', 120)
            UNION ALL
            SELECT CONVERT([datetime], '2023-04-07 18:25:00', 120)
            UNION ALL
            SELECT CONVERT([datetime], '2023-04-07 17:14:12', 120)
            UNION ALL
            SELECT CONVERT([datetime], '2023-04-06 18:11:11', 120)
            UNION ALL
            SELECT CONVERT([datetime], '2023-04-06 16:01:35', 120)
            UNION ALL
            SELECT CONVERT([datetime], '2023-04-06 17:45:52', 120)
          ) t
      ) t2
  ) t3
ORDER BY
  [t],
  [N]