RangeValidator

<form id="form1" runat="server">
    <div>
        Arrival Date:
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>&nbsp;
        <asp:RangeValidator ID="RangeValidator1" runat="server"
         Text="You must only select a date within the next two weeks."
         ControlToValidate="TextBox1" Type="Date"></asp:RangeValidator><br />
        <br />
        Select your arrival date:<br />
        <asp:Calendar ID="Calendar1" runat="server"
   ...