Hi,
Try setting the
PopUpButtonId
of
CalanderExtender
as your TextBox ID.
Try this:
<ajaxtoolkit:calendar runat="server" xmlns:ajaxtoolkit="#unknown">
TargetControlID="TextBox1"
CssClass="ClassName"
Format="MMMM d, yyyy"
PopupButtonID="TextBox1" /></ajaxtoolkit:calendar>
--Amit