using System;

using System.Collections.Generic;

using System.Linq;

public class Person

{

    public string Name { get; set; }

    public List<DateTime> VacationDays { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        var people = new List<Person>

        {

            new Person

            { 

                Name = "David", 

                VacationDays = new List<DateTime>

                { 

                    DateTime.Parse("1/1/2019"), 

                    DateTime.Parse("1/2/2019") 

                }

            },

            new Person

            { 

                Name = "Bob", 

                VacationDays = new List<DateTime>

                {

                    DateTime.Parse("1/1/2019"),

                    DateTime.Parse("1/3/2019") 

                }

            }

        };

        var flatList = people.SelectMany

        (

            p => p.VacationDays.Select( d => new { Person = p, Date = d } )

        );

​

        Console.WriteLine("Flat list:");

        foreach (var entry in flatList)

        {

            Console.WriteLine("{0} has the day off on {1:MM/dd/yyyy}", entry.Person.Name, entry.Date);

        }

​

        var lookFor = DateTime.Parse("1/1/2019");

        var entriesForJan01 = flatList.Where( f => f.Date == lookFor );

        Console.WriteLine("People with a vacation on 1/1/2019:");

        foreach (var entry in entriesForJan01)

        {

            Console.WriteLine("{0} has the day off on {1:MM/dd/yyyy}", entry.Person.Name, entry.Date);

        }

    }

}