<!DOCTYPE html>
<html>
<head>
<script  runat="server">
Sub DaySelect(obj As Object, e As DayRenderEventArgs)
      If e.Day.IsWeekend Then
           e.Day.IsSelectable = False
      End If
End Sub
</script>

</head>

<body>
      <form runat="server">
      The weekends are not selectable
      <asp:Calendar id="cal1" runat="server" OnDayRender="DaySelect"/>
</form>
</body>
</html>