using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public class myclass

    {

        public DateTime SomeDate;

        public string SomeString;

        public override string ToString()

        {

            return $"{SomeString}: {SomeDate}";

        }

    }

    public static void Main()

    {

        // populate with your sample data

        var L = new List<List<myclass>>();

        List<myclass> t = new List<myclass>();

        var mc = new myclass() { SomeString = "5", SomeDate = DateTime.Now };

        myclass mc1 = new myclass() { SomeString = "12", SomeDate = DateTime.Now.AddDays(1) };

        myclass mc2 = new myclass() { SomeString = "123", SomeDate = DateTime.Now.AddDays(2) };

        myclass mc3 = new myclass() { SomeString = "77", SomeDate = DateTime.Now.AddDays(3) };

        myclass mc4 = new myclass() { SomeString = "882", SomeDate = DateTime.Now.AddDays(4) };

        t.Add(mc);

        t.Add(mc1);

        t.Add(mc2);

        t.Add(mc3);

        t.Add(mc4);

        L.Add(t);

        List<myclass> t2 = new List<myclass>();

        myclass mc5 = new myclass() { SomeString = "166", SomeDate = DateTime.Now.AddDays(500) };

        myclass mc6 = new myclass() { SomeString = "344", SomeDate = DateTime.Now.AddDays(501) };

        myclass mc7 = new myclass() { SomeString = "123", SomeDate = DateTime.Now.AddDays(502) };

        myclass mc8 = new myclass() { SomeString = "234", SomeDate = DateTime.Now.AddDays(503) };

        myclass mc9 = new myclass() { SomeString = "123", SomeDate = DateTime.Now.AddDays(504) };

        t2.Add(mc5);

        t2.Add(mc6);

        t2.Add(mc7);

        t2.Add(mc8);

        t2.Add(mc9);

        L.Add(t2);

        var target1 = "123";

        var date = DateTime.Now.AddDays(502).Date;

        // END populate with your sample data

        // My 2 approaches:

        // 1: If you want the first list which contain at least one object that match your predicate:

        var result = L.Where(list => list.Any(x => x.SomeString == target1 && x.SomeDate.Date == date)).FirstOrDefault();

​

​

        // 2: If you want the lists which contain at least one object that match your predicate 

        //    and those list should only contain those objects, so all other non-matching objects are filtered out: 

        var result2 = L

            .Select(list => list.Where(x => x.SomeString == target1 && x.SomeDate.Date == date).ToList())

            .Where(list => list.Any())

            .ToList();

        // Output results:

        Console.WriteLine("Approach #1");

        for(int i = 0; i < result.Count; i++)

            Console.WriteLine($"Object#{i+1}: {string.Join(",", result[i])}");

        Console.WriteLine();

        Console.WriteLine("Approach #2");

        for(int i = 0; i < result2.Count; i++)

            Console.WriteLine($"List#{i+1}: {string.Join(",", result2[i])}");

    }

}