je code

using Groenbos.Models;
using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.DataVisualization.Charting;
using System.Web.UI.WebControls;

namespace Groenbos.Recreatie
{
    public partial class VerhuurStatistieken : System.Web.UI.Page
    {
        private GroenbosContext db = new GroenbosContext();

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                CultureInfo myCI = new CultureInfo("en-US");
                System.Globalization.Calendar myCal = myCI.Calendar;

                // Gets the DTFI properties required by GetWeekOfYear.
                CalendarWeekRule myCWR = myCI.DateTimeFormat.CalendarWeekRule;
                DayOfWeek myFirstDOW = myCI.DateTimeFormat.FirstDayOfWeek;

                int weekn = myCal.GetWeekOfYear(DateTime.Now, myCWR, myFirstDOW);
                VulGrafiek(DateTime.Today.Year,weekn);
                List<int> weeks = new List<int>();
                for(int i =0; i <= 52; i++)
                {
                    weeks.Add(i);
                }
                foreach(var w in weeks)
                {
                    var item = new ListItem
                    {
                        Text = w.ToString(),
                        Value = w.ToString()
                    };
                    ddlWeek.Items.Add(item);
                }
                ddlWeek.SelectedValue = weeks[weekn].ToString();
            }



        }

        private void VulGrafiek(int jaar, int weekNummer)
        {
            List<RecreatieOrder> orders = db.RecreatieOrder.ToList();
            DateTime now = DateTime.Today;
            Series series = aVChart.Series["Series1"];
            Series series2 = aVChart.Series["Series2"];
            aVChart.Series["Series1"].IsValueShownAsLabel = true;
            aVChart.Series["Series2"].IsValueShownAsLabel = true;
            decimal totaal = 0;
            decimal totaal2 = 0;

            var week = GetWeekDaysOfWeekFrom(weekNummer, jaar);
            foreach (var dag in week)
            {
                List<RecreatieOrder> reserveringen = orders.Where(x => x.Datum.Day == dag.Day && x.Datum.Year == int.Parse(ddlJaar.SelectedValue)).ToList();
                List<RecreatieOrder> reserveringen2 = orders.Where(x => x.Datum.Day == dag.Day && x.Datum.Year == (int.Parse(ddlJaar.SelectedValue))-1).ToList();
                foreach (var r in reserveringen)
                {
                    totaal += 5;
                }
                foreach (var r in reserveringen2)
                {
                    totaal2 += 5;
                }
                series.Points.AddXY(dag, reserveringen.Count());
                series2.Points.AddXY(dag, reserveringen2.Count());
            }

            lblOmzet.Text = "De totale omzet van deze maand is: €" + Math.Round(totaal, 2).ToString();
            lblOmzet2.Text = "De totale omzet van deze maand vorig jaar was: €" + Math.Round(totaal2, 2).ToString();
        }

        protected void ddlJaar_SelectedIndexChanged(object sender, EventArgs e)
        {
            VulGrafiek(int.Parse(ddlJaar.SelectedValue), int.Parse(ddlWeek.SelectedValue));
        }

        protected void ddlWeek_SelectedIndexChanged(object sender, EventArgs e)
        {
            VulGrafiek(int.Parse(ddlJaar.SelectedValue), int.Parse(ddlWeek.SelectedValue));
        }

        public List<DateTime> GetWeekDaysOfWeekFrom(int weekNumber, int jaar)
        {

            DateTime firstDayOfWeek = FirstDateOfWeekISO8601(jaar, weekNumber);

            int currentDayOfWeek = (int)firstDayOfWeek.DayOfWeek;
            DateTime sunday = firstDayOfWeek.AddDays(-currentDayOfWeek);
            DateTime monday = sunday.AddDays(1);
            // If we started on Sunday, we should actually have gone *back*
            // 6 days instead of forward 1...
            if (currentDayOfWeek == 0)
            {
                monday = monday.AddDays(-7);
            }

            return Enumerable.Range(0, 7).Select(days => monday.AddDays(days)).ToList();
        }

        public static DateTime FirstDateOfWeekISO8601(int year, int weekOfYear)
        {
            DateTime jan1 = new DateTime(year, 1, 1);
            int daysOffset = DayOfWeek.Thursday - jan1.DayOfWeek;

            DateTime firstThursday = jan1.AddDays(daysOffset);
            var cal = CultureInfo.CurrentCulture.Calendar;
            int firstWeek = cal.GetWeekOfYear(firstThursday, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

            var weekNum = weekOfYear;
            if (firstWeek <= 1)
            {
                weekNum -= 1;
            }
            var result = firstThursday.AddDays(weekNum * 7);
            return result.AddDays(-3);
        }

        protected void btnNext_Click(object sender, EventArgs e)
        {

        }

        protected void btnPrevious_Click(object sender, EventArgs e)
        {

        }
    }
}