using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<Player> players = new List<Player>

        {

            new Player{Date = new DateTime(2021, 1, 1), ID = 23}, 

            new Player{Date = new DateTime(2021, 1, 1), ID = 29}, 

            new Player{Date = new DateTime(2021, 1, 1), ID = 24}, 

            new Player{Date = new DateTime(2021, 1, 3), ID = 22}, 

            new Player{Date = new DateTime(2021, 1, 3), ID = 23}

        };

        var startDate = players.OrderBy(x => x.Date)

            .First()

            .Date;

        var endDate = players.OrderBy(x => x.Date)

            .Last()

            .Date;

        var dates = Enumerable.Range(0, 1 + endDate.Subtract(startDate).Days)

          .Select(offset => startDate.AddDays(offset))

          .ToArray(); 

        var result = (from a in dates

                      join b in 

                      (

                         from p in players

                         group p by p.Date into g

                         select new { Date = g.Key, Count = g.Count() }

                      ) on a.Date equals b.Date into ab

                      from b in ab.DefaultIfEmpty()

                      select new { Date = a.Date, Count = b != null ? b.Count : 0 }

                      ); 

        foreach (var item in result)

        {

            Console.WriteLine(String.Format("Date: {0}, Count: {1}", item.Date.ToString("d/M/yyyy"), item.Count));

        }

    }

}

​

public class Player

{

    public DateTime Date { get; set; }

​

    public int ID { get; set; }

}