Below code check date selected by U through calander control,if it already passed then it show alert date already passed else show date in textbox...............
IN .aspx page :
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Calendar ID="Calendar1" runat="server" OnSelectionChanged="Calendar1_SelectionChanged1" ></asp:Calendar>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
</ContentTemplate>
</asp:UpdatePanel>
</div>
In .vb page
Protected Sub Calendar1_SelectionChanged1(ByVal sender As Object, ByVal e As System.EventArgs)
If Calendar1.SelectedDate < System.DateTime.Now Then
Page.ClientScript.RegisterStartupScript(Me.GetType(), "amit", "alert('Date Already passed');", True)
Exit Sub
Else
TextBox1.Text = Calendar1.SelectedDate
End If
End Sub