Try:
<ajaxToolkit:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="Date1">
</ajaxToolkit:CalendarExtender>
Update:
Do you have the following in your web.config?
<pages>
<controls>
<add tagPrefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" tagPrefix="asp" />
</controls>
</pages>
<compilation>
<assemblies>
<add assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add assembly="System.Web.Extensions.Design, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
</assemblies>
</compilation>
<httpHandlers>
<remove verb="*" path="*.asmx" />
<add verb="*" path="*.asmx" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add verb="*" path="*_AppService.axd" validate="false" type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" validate="false" />
</httpHandlers>
<httpModules>
<add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
</httpModules>
Update II
Put the following at the top of your aspx page.
<%@ Register Tagprefix="asp" Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit" %>
Also, you're referencing AjaxControlToolkit.dll, right? It should be in your bin directory. Right-click project, Add Reference menu choice to add.