Call OnChange along with selectedindexchanged in dropdownlist

<script type="text/javascript">
    function Validate(Uid)
    {      
        document.getElementById('<%=TBPwd1.ClientID%>').value = '';
        document.getElementById('<%=TBPwd2.ClientID%>').value = '';
        eval('__doPostBack("'+Uid+'", "");');
        return false;
    }
</script>


<asp:DropDownList ID="DropDownListLevel" runat="server" AutoPostBack="True"  Width="250px" onchange="return Validate(this.name);"  onselectedindexchanged="DropDownListLevel_SelectedIndexChanged">
    </asp:DropDownList>


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

Check and Uncheck all the checkbox in the grid and check wheather any checkbox is checked or not

    <script type="text/javascript">
        // Select/Deselect checkboxes based on header checkbox
        function SelectheaderCheckboxes(headerchk) {

            var gvcheck = document.getElementById('<%=grd_Approve.ClientID%>');
            var i;
            //Condition to check header checkbox selected or not if that is true checked all checkboxes
            if (headerchk.checked) {
                for (i = 0; i < gvcheck.rows.length; i++) {
                    var inputs = gvcheck.rows[i].getElementsByTagName('input');
                    inputs[0].checked = true;
                }
            }
            //if condition fails uncheck all checkboxes in gridview
            else {
                for (i = 0; i < gvcheck.rows.length; i++) {
                    var inputs = gvcheck.rows[i].getElementsByTagName('input');
                    inputs[0].checked = false;
                }
            }
        }
        //function to check header checkbox based on child checkboxes condition
        function Selectchildcheckboxes(header) {
            var ck = header;
            var count = 0;
            var gvcheck = document.getElementById('<%=grd_Approve.ClientID%>');
            var headerchk = document.getElementById(header);
            var rowcount = gvcheck.rows.length;
            //By using this for loop we will count how many checkboxes has checked
            if (rowcount > 22) {
                for (i = 1; i < gvcheck.rows.length - 1; i++) {
                    var inputs = gvcheck.rows[i].getElementsByTagName('input');

                    if (inputs[0].checked) {
                        count++;
                    }
                }
            }
            else {
                for (i = 1; i < gvcheck.rows.length; i++) {
                    var inputs = gvcheck.rows[i].getElementsByTagName('input');

                    if (inputs[0].checked) {
                        count++;
                    }
                }
            }

            //Condition to check all the checkboxes selected or not
            if (count == rowcount - 1) {
                headerchk.checked = true;
            }
            else {
                headerchk.checked = false;
            }
        }
</script>

<script type="text/javascript">
    function ConfirmInbox() {

        var count = 0;
        var gvcheck = document.getElementById('<%=grd_Approve.ClientID%>');
        var rowcount = gvcheck.rows.length;
        if (rowcount > 22) {
            for (i = 1; i < gvcheck.rows.length - 1; i++) {
                var inputs = gvcheck.rows[i].getElementsByTagName('input');

                if (inputs[0].checked) {
                    count++;
                }
            }

        }
        else {
            for (i = 1; i < gvcheck.rows.length; i++) {
                var inputs = gvcheck.rows[i].getElementsByTagName('input');

                if (inputs[0].checked) {
                    count++;
                }
            }
        }
        if (count > 0) {
            var r = confirm("Are you sure You want to Approve ?")
            if (r == true) {
                return true;
            }
            else {
                return false;
            }
        }
        else {
            alert("Please Select atleast one checkbox to Approve.");
            return false;

        }
    }

    </script>



<asp:GridView ID="grd_Approve" runat="server" Width="100%"
            AutoGenerateColumns="False" AllowPaging="True" PageSize="20"
            onpageindexchanging="grd_Approve_PageIndexChanging"
            onrowcommand="grd_Approve_RowCommand" ShowFooter="True" DataKeyNames="S_no"
            onrowdatabound="grd_Approve_RowDataBound">
    <Columns>
    <asp:BoundField DataField="SNo" HeaderText="SNo" >
   
       <HeaderStyle HorizontalAlign="Left"></HeaderStyle>
        </asp:BoundField>
   
    <asp:TemplateField HeaderText="Staging">
        <ItemTemplate>
            <asp:Label ID="lbl_Name" runat="server" Text='<%#Eval("StageName") %>'></asp:Label>
        </ItemTemplate>      

<HeaderStyle HorizontalAlign="Left"></HeaderStyle>
    </asp:TemplateField>

    <asp:TemplateField HeaderText="Infestation">
        <ItemTemplate>
            <asp:Label ID="lbl_Name" runat="server" Text='<%#Eval("Infestation_Nm") %>'></asp:Label>
        </ItemTemplate>      

<HeaderStyle HorizontalAlign="Left"></HeaderStyle>
    </asp:TemplateField>

    <asp:TemplateField HeaderText="Image">
        <ItemTemplate>
            <asp:ImageButton ID="img_Symptoms" runat="server" ImageUrl='<%# "images/Symptoms/" + Eval("ImageUrl") %>' />
        </ItemTemplate>      

    <HeaderStyle HorizontalAlign="Left"></HeaderStyle>
    </asp:TemplateField>

    <asp:TemplateField HeaderText="Symptoms">
        <ItemTemplate>
            <asp:Label ID="lbl_Name" runat="server" Text='<%#Eval("Symptoms") %>'></asp:Label>
        </ItemTemplate>      

<HeaderStyle HorizontalAlign="Left"></HeaderStyle>
    </asp:TemplateField>

    <asp:TemplateField>
    <HeaderTemplate>
    <asp:CheckBox ID="chkheader" Text="All" runat="server" onclick="javascript:SelectheaderCheckboxes(this)" />
   </HeaderTemplate>
        <ItemTemplate>
           <asp:CheckBox ID="cbox_Delete" runat="server" />
        </ItemTemplate>
      <FooterTemplate>
    <asp:Button ID="btn_ApproveMultiple" runat="server" Text="Delete" CommandName="Approve"  OnClientClick="return ConfirmInbox();" />
    </FooterTemplate>  

<HeaderStyle HorizontalAlign="Left"></HeaderStyle>
    </asp:TemplateField>

   
 
    </Columns>
    <PagerStyle   HorizontalAlign="Left" VerticalAlign="Middle" />
             <PagerSettings  Mode="NextPreviousFirstLast"
             FirstPageText="<<"
             PreviousPageText="<"
             NextPageText=">"
             LastPageText=" >>"
             FirstPageImageUrl="~/Advs/images/First.png"
             PreviousPageImageUrl="~/Advs/images/Previous.png"
             NextPageImageUrl="~/Advs/images/Next.png"
             LastPageImageUrl="~/Advs/images/Last.png"   />
    </asp:GridView>

Disable Future Dates in Calendar Extendar

http://aspsnippets.com/Articles/Disable-Future-Dates-in-ASPNet-AJAX-Calendar-Extender-Control.aspx