Поправка, вариант 1

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplicationDemo
{
    public partial class Popravka : System.Web.UI.Page
    {
        // 1
        protected void Page_Load(object sender, EventArgs e)
        {
            Calendar1.Visible = false;
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            if (TextBox1.Text == "Календар")
            {
                Calendar1.Visible = true;
            }
            else
            {
                TextBox1.Text = "Въведете текст - Календар";
            }
        }

        // 2
        protected void Calendar1_SelectionChanged(object sender, EventArgs e)
        {
            var date = Calendar1.SelectedDate;
            for (int i = 0; i <= 7; i++)
            {
                ListBox1.Items.Add(new ListItem(date.ToShortDateString()));
                date = date.AddDays(1);
            }
        }

        // 4
        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            Session["destinationType"] = DropDownList1.SelectedValue;
            Response.Redirect("PopravkaPage2.aspx");
        }
    }
}