Why does hiding a div that has an ajax (Popupcontrolextender for calendar) in page_load causes an RegisterDataItem error?

Error message saying "RegisterDataItem can only be called during an async postback."  How do i fix this?  The error only triggers if a date has been selected and you wish to change it by selecting it again on the same textbox

I'm trying to make calendar pops up upon clicking on a textbox using the popupcontrol from the Ajax Control Toolkit but it causes an error because I hide the div element at page_load.  I really need to hide the div as it only appears base on a button's logic and status.  Any way around this?  I'm lost, i've tried googling the answers for hours!  xhtmlConformance = transitional, nesting, etc etc.  No luck

This is what I have for my calendar ajax:

<asp:TextBox ID="txtARDate" runat="server"></asp:TextBox><cc1:PopupControlExtender ID="txtARDate_PopupControlExtender" runat="server" 
                        DynamicServicePath="" Enabled="True" ExtenderControlID="" Position="Bottom" 
                        TargetControlID="txtARDate" PopupControlID="pARDate"></cc1:PopupControlExtender><asp:Panel ID="pARDate" runat="server"><asp:UpdatePanel ID="UpdatePanel1" runat="server"><ContentTemplate><asp:Calendar ID="calARDate" runat="server" BackColor="White" 
                                BorderColor="#3366CC" BorderWidth="1px" CellPadding="1" 
                                DayNameFormat="Shortest" Font-Names="Verdana" Font-Size="8pt" 
                                ForeColor="#003399" Height="200px" 
                                onselectionchanged="calARDate_SelectionChanged" Width="220px"><DayHeaderStyle BackColor="#99CCCC" ForeColor="#336666" Height="1px" /><NextPrevStyle Font-Size="8pt" ForeColor="#CCCCFF" /><OtherMonthDayStyle ForeColor="#999999" /><SelectedDayStyle BackColor="#009999" Font-Bold="True" ForeColor="#CCFF99" /><SelectorStyle BackColor="#99CCCC" ForeColor="#336666" /><TitleStyle BackColor="#003399" BorderColor="#3366CC" BorderWidth="1px" 
                                    Font-Bold="True" Font-Size="10pt" ForeColor="#CCCCFF" Height="25px" /><TodayDayStyle BackColor="#99CCCC" ForeColor="White" /><WeekendDayStyle BackColor="#CCCCFF" /></asp:Calendar></ContentTemplate></asp:UpdatePanel></asp:Panel>