Making a Validiation, and try to display dates from the list

Question

I maiding a Validiation to my code, and I want to Find Out If a dates from the list are in my Textbox. But I can't view them. How should I List them. I tried Label2.Text += olistaDniSwiatecznych["Data"].ToString(); But nothing happend. Whot schould I write in the Query. Here is all my botton Click method:

 protected void Button2_Click(object sender, EventArgs e)
            {
                SPWeb witryna = SPContext.Current.Web;



                    // Dodawanie poszczególnych dni projektowych
                    SPListItemCollection listItems = witryna.Lists["Szczegoly"].Items;
                    SPListItem item = listItems.Add();
                    item["Kontrakt"] = TextBox1.Text;
                    item["Pracownik"] = TextBox2.Text;
                    item["Dzien"] = TextBox3.Text;
                    var selectedGroup = DropDownList2.SelectedItem;
                    item["Procenty"] = new SPFieldLookupValue(int.Parse(selectedGroup.Value), selectedGroup.Text);

                    string DateOnlyQuery = "<Where><Eq><FieldRef Name='{0}' /><Value Type='DateTime'>{1}</Value></Eq></Where>";
                    string dateISO = SPUtility.CreateISO8601DateTimeFromSystemDateTime(Convert.ToDateTime(TextBox3.Text).ToUniversalTime().AddHours(2.0));

                    var query2 = new SPQuery();
                    query2.Query = String.Format(DateOnlyQuery, "Data", dateISO);
                    query2.ViewFields = "<FieldRef Name='Data' />";
                    query2.ViewFieldsOnly = true;
                    SPListItemCollection listaDniSwiatecznych = witryna.Lists["DR2013 Święta"].GetItems(query2);

                    foreach (SPListItem olistaDniSwiatecznych in listaDniSwiatecznych)
                    {
                        Label2.Text += olistaDniSwiatecznych["Data"].ToString();

                    }

                    // Zaktualizowanie wpisu w liscie projektów
                    item.Update();
                    DropDownList2.SelectedValue = "2";
                    this.TextBox1.Text = "";
                    this.TextBox2.Text = "";
                    this.TextBox3.Text = "";
                    Label3.Text = "<span style='color:red; font-size:13px; font-weight:bold;'>Nowy wpis został dodany</span>";



            }

If I done like this

                    SPListItemCollection listaDniSwiatecznych = witryna.Lists["DR2013 Święta"].Items;

                     foreach (SPListItem olistaDniSwiatecznych in listaDniSwiatecznych)
                     {
                         Label2.Text += olistaDniSwiatecznych["Data"].ToString();
                     }

, it's work perfect, but I want to faster do this by CAML.