Retain values of List between page postbacks

Question

I am working on this code and I realised that the List gets initialized everytime from a different event handler. I do not want the List to be initialized to empty, I need the value to be retained. I have placed the declaration of the list in the class instead of in the method. Please go through my code and correct me. 

Hi, I have edited my post, did some reading and realised that it is very much connected to retaining values between page postbacks. 

public partial class WebForm4 : System.Web.UI.Page
   {
       #region List of Weeks
       public List A_Week_1 = new List();
       public List A_Week_2 = new List();
       public List A_Week_3 = new List();
       public List A_Week_4 = new List();
       public List A_Week_5 = new List();
       public List A_Week_6 = new List();
       #endregion





       protected void Page_Load(object sender, EventArgs e)
       {



       }
       protected void btnUpload_Click(object sender, EventArgs e)
       {

           if (FileUpload1.HasFile)
           {
               try
               {
                   string filename = Path.GetFileName(FileUpload1.FileName);
                   //FileUpload1.SaveAs(Server.MapPath("~/Uploaded Files/") + filename);
                   string fileExtension = Path.GetExtension(FileUpload1.PostedFile.FileName);

                   string fileLocation = Server.MapPath("~/Uploaded Files/" + filename);
                   FileUpload1.SaveAs(fileLocation);
                   lblMessage.Text = "Upload status: File uploaded to " + fileLocation;

                   readfile();
               }
               catch (Exception ex)
               {
                   lblMessage.Text = "Upload status: The file could not be uploaded. The following error occured: " + ex.Message;
               }
           }
       }
       protected void readfile()
       {
           //datasource
           string connString = ConfigurationManager.ConnectionStrings["testexcel"].ConnectionString;

           //object represents a unique connection to a data source.
           OleDbConnection oledbConn = new OleDbConnection(connString);

           try
           {
               // Open connection
               oledbConn.Open();
               DataTable dtsheet = new DataTable("storesheettable");
               //because the oledbConn.GetOlddbSchemaTable returns datatale
               dtsheet = oledbConn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);

               //initialize and call constructor
               if (dtsheet == null)
               {
                   Label1.Text = "No sheet to display";
               }
               else
               {

                   //String[] excelSheets = new String[dtsheet.Rows.Count];
                   List<string> import_excel_weeks_list = new List<string>();
                   //int i = 0;


                   char[] charsToTrim = { '$' };
                   for (int i = 0; i < dtsheet.Rows.Count; i++)
                   {
                       //excelSheets[i] = dtsheet.Rows.ToString();
                       import_excel_weeks_list.Add(dtsheet.Rows[i]["TABLE_NAME"].ToString().Trim(charsToTrim));


                   }

                   Repeater1.DataSource = import_excel_weeks_list;
                   Repeater1.DataBind();
               }


           }
           catch (Exception ex)
           {
               catchexception.Text = ex.ToString();

           }
           finally
           {
               // Close connection
               oledbConn.Close();
           }

       }
       protected void checkcalendarweek(string weekname)
       {
           if (weekname == "Week2")
           {
               int storemonth;
               DateTime today = new DateTime();
               storemonth = today.Month;
               Response.Write(storemonth);


           }
       }
       public void monthsddl_SelectedIndexChanged(object sender, EventArgs e)
       {
           int total_no_days;
           int selected_month = Convert.ToInt32(monthsddl.SelectedValue);

           //total number of days in the month for the year
           total_no_days = DateTime.DaysInMonth(DateTime.Today.Year, selected_month);
           Label1.Text = String.Format("Total number of days in the month of {0} is {1}", monthsddl.SelectedItem,total_no_days);


           DateTime firstOfMonth = new DateTime(Convert.ToInt32(DateTime.Now.Year), selected_month, 1);
           int startdayofthemonth = (int)DayOfWeek.Monday;
           //total weeks in a month
           int weeksInMonth = (int)Math.Ceiling((startdayofthemonth + total_no_days) / 7.0);



           int weeknum = 0;

           //dates are of course from 1 to the total_no_days
           for (int i = 1; i <= total_no_days; i++)
           {
               //create the date based on the selected month with i starting as 1
               DateTime date1 = new DateTime(DateTime.Today.Year, selected_month, i);
               DateTime dateonly = date1.Date;
               //get the local culture timeformat
               DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;

               //gets the calendar for the local timeformat and assign it to a type of calendar, gregorian
               System.Globalization.Calendar cal = dfi.Calendar;
               int num = cal.GetWeekOfYear(date1, dfi.CalendarWeekRule, (DayOfWeek.Monday));

               //week in a month
               DateTime test = new DateTime(date1.Year, date1.Month, 1);
               int num2 = cal.GetWeekOfYear(test, dfi.CalendarWeekRule, (DayOfWeek.Monday));
               weeknum = num - num2 + 1;

               //input dates for the week
               if (weeknum == 1)
               {
                   A_Week_1.Add(date1);
               }
               else if(weeknum == 2)
               {
                   //week2.Add(new Week() { datefortheweek = date1 });
                   A_Week_2.Add(date1);
               }
               else if(weeknum == 3)
               {
                   //week3.Add(new Week() { datefortheweek = date1 });
                   A_Week_3.Add(date1);
               }
               else if (weeknum == 4)
               {
                   //week4.Add(new Week() { datefortheweek = date1 });
                   A_Week_4.Add(date1);
               }
               else if (weeknum == 5)
               {
                   //week5.Add(new Week() { datefortheweek = date1 });
                   A_Week_5.Add(date1);
               }
               else
               {
                   //week6.Add(new Week() { datefortheweek = date1 });
                   A_Week_6.Add(date1);
               }



               Response.Write(weeknum);


               Response.Write(" ");
               Response.Write(dateonly.ToString("dd/M/yyyy"));
               Response.Write(" ");
               Response.Write(dateonly.DayOfWeek);
               Response.Write("");

           }




           //displaying the list
           foreach (DateTime text in A_Week_5)
           {
               Response.Write(text);
               Response.Write("");
           }
           //Response.Write("Test" + A_Week_5.Last().datefortheweek);
           Response.Write("Test" + A_Week_5.Last());
           //createtable(weeksInMonth, week1.First().datefortheweek, week1.Last().datefortheweek);
           createtable(weeknum, A_Week_1, A_Week_2, A_Week_3, A_Week_4, A_Week_5, A_Week_6);
       }

       protected void createtable(int pass_in_weeksInMonth, List week_1, List week_2, List week_3, List week_4, List week_5, List week_6)
       {

           DataTable createweek_dates = new DataTable();
           DataColumn column_createweek_dates;
           DataRow rows_createweek_dates;

           //Column 1
           column_createweek_dates = new DataColumn();
           column_createweek_dates.DataType = Type.GetType("System.Int32");
           column_createweek_dates.ColumnName = "Week Number";
           column_createweek_dates.ReadOnly = true;
           column_createweek_dates.Unique = true;
           // Add the Column to the table
           createweek_dates.Columns.Add(column_createweek_dates);

           //Column 2
           column_createweek_dates = new DataColumn();
           column_createweek_dates.DataType = Type.GetType("System.DateTime");
           column_createweek_dates.ColumnName = "Start Date";
           column_createweek_dates.ReadOnly = true;
           //column_createweek_dates.Unique = true;
           // Add the Column to the table
           createweek_dates.Columns.Add(column_createweek_dates);

           //Column 3
           column_createweek_dates = new DataColumn();
           column_createweek_dates.DataType = Type.GetType("System.DateTime");
           column_createweek_dates.ColumnName = "End Date";
           column_createweek_dates.ReadOnly = true;
           //column_createweek_dates.Unique = true;
           // Add the Column to the table
           createweek_dates.Columns.Add(column_createweek_dates);

           for (int i = 1; i <= pass_in_weeksInMonth; i++)
           {
               //new row for every creation
               rows_createweek_dates = createweek_dates.NewRow();
               rows_createweek_dates["Week Number"] = i;
               for (int j = i; j <= i; j++)
               {
                   if (j == 1)
                   {
                       rows_createweek_dates["Start Date"] = week_1.First();
                       rows_createweek_dates["End Date"] = week_1.Last();
                   }
                   else if (j == 2)
                   {
                       rows_createweek_dates["Start Date"] = week_2.First();
                       rows_createweek_dates["End Date"] = week_2.Last();
                   }
                   else if (j == 3)
                   {
                       rows_createweek_dates["Start Date"] = week_3.First();
                       rows_createweek_dates["End Date"] = week_3.Last();
                   }
                   else if (j == 4)
                   {
                       rows_createweek_dates["Start Date"] = week_4.First();
                       rows_createweek_dates["End Date"] = week_4.Last();
                   }
                   else if (j == 5)
                   {
                       rows_createweek_dates["Start Date"] = week_5.First();
                       rows_createweek_dates["End Date"] = week_5.Last();
                   }
                   else
                   {
                       rows_createweek_dates["Start Date"] = week_6.First();
                       rows_createweek_dates["End Date"] = week_6.Last();
                   }

               }

               createweek_dates.Rows.Add(rows_createweek_dates);
           }

           rptItemsInCart.DataSource = createweek_dates;
           rptItemsInCart.DataBind();

       }
       public void DoSend(object sender, EventArgs e)
       {
           AllLists create1 = new AllLists();
           for (int i = 0; i < rptItemsInCart.Items.Count; i++)
           {
               //Retrieve the state of the CheckBox
               CheckBox cb = (CheckBox)rptItemsInCart.Items[i].FindControl("CheckBox1");
               if (cb.Checked)
               {
                   //Retrieve the value associated with that CheckBox
                   HiddenField hiddentext = (HiddenField)rptItemsInCart.Items[i].FindControl("HiddenField1");

                   //Now we can use that value to do whatever we want
                   Label2.Text = hiddentext.Value;
                   create1.user_select_weeks_list.Add(hiddentext.Value);
               }
           }

           selected_weeks_repeater.DataSource = create1.user_select_weeks_list;
           selected_weeks_repeater.DataBind();

           foreach(DateTime dates1 in A_Week_1)
           {
               Response.Write(dates1);
           }

       }
       public void Week_button_Click(object sender, EventArgs e)
       {
           DataTable excel_data_gd = new DataTable();
           DataColumn edg_column;
           DataRow edg_row;

           //get the data from repeater control
           AllLists create1 = new AllLists();
           for(int i = 0; i < selected_weeks_repeater.Items.Count; i++)
           {
               Label pass_as_label = (Label)selected_weeks_repeater.Items[i].FindControl("store_user_sheet");
               create1.user_select_weeks_list.Add(pass_as_label.Text);
           }

           //for(int j = 0; j < create1.user_select_weeks_list.Count; j++)
           //{
           //    Response.Write(create1.user_select_weeks_list[j]);
           //}


           // Column1 - Machine Name
           edg_column = new DataColumn();
           edg_column.DataType = System.Type.GetType("System.String");
           edg_column.ColumnName = "Machine Name";
           edg_column.ReadOnly = true;
           edg_column.Unique = true;
           // Add the Column to the DataColumnCollection.
           excel_data_gd.Columns.Add(edg_column);

           string[] machinename_array1 = Import_xml_file();
           for (int i = 0; i < machinename_array1.Length; i++)
           {
               edg_row = excel_data_gd.NewRow();
               edg_row["Machine Name"] = machinename_array1[i];
               //import from excel the value
               excel_data_gd.Rows.Add(edg_row);
           }

           //check if the user has selected any value or not
           if(create1.user_select_weeks_list.Contains("Week2"))
           {
               //check got how many dates within the week

               int count_of_dates = A_Week_2.Count;
               // The dates column, create a new column for every date within the week
              for(int i = 0; i < count_of_dates; i++)
               {
                   edg_column = new DataColumn();
                   edg_column.DataType = System.Type.GetType("System.DateTime");
                   edg_column.ColumnName = A_Week_2[i].ToString();
                   edg_column.ReadOnly = true;
                   edg_column.Unique = true;
                   // Add the Column to the DataColumnCollection.
                   excel_data_gd.Columns.Add(edg_column);
               }
           }







           GridView1.DataSource = excel_data_gd;
           GridView1.DataBind();

       }
       protected string[] Import_xml_file()
       {
           XmlDocument doc = new XmlDocument();
           doc.Load(Server.MapPath("~/App_Data/List_of_machines.xml"));
           //Display all the Activities alone in the XML
           XmlNodeList elemList = doc.GetElementsByTagName("machine_name");

           string[] machinename_array = new string[elemList.Count];
           for (int i = 0; i < elemList.Count; i++)
           {
               machinename_array[i] = elemList[i].InnerXml;
           }

           return machinename_array;


       }
   }

What is the best way of retaining the values, ViewState or Session, I read that viewstate if used with large data can cause the page to load slower, how can I go about solving this.