asp:Calender in asp:UpdatePanel triggers Autopostback :(

Hello everyone,
I have a small problem with my asp:Calender. On my Page I have some Textboxes and Checkboxes and in the end a Calender.
All of them are prefilled with Info's at the first load of the page.
But, if I mark a new date at the calender for the first time a autopostback is triggered and all my previos inputs are cleared.
<asp:UpdatePanel ID="UpdatePanel1" runat="server"> <ContentTemplate> <asp:Calendar runat="server" ID="Calendar1" BackColor="White" BorderColor="Black" DayNameFormat="Shortest" Font-Names="Times New Roman" Font-Size="10pt" ForeColor="Black" Height="220px" NextPrevFormat="FullMonth" TitleFormat="Month" Width="400px"> <DayHeaderStyle BackColor="#CCCCCC" Font-Bold="True" Font-Size="7pt" ForeColor=&q ...