<!DOCTYPE html>
<html>
<head>
    <script  runat="server">
         Sub ButtonClick(obj As Object, e As EventArgs)
             cal1.VisibleDate = New DateTime(cal1.TodaysDate.Year, ddl1.SelectedIndex + 1, 1)
         End Sub 'ButtonClick
    </script>

</head>
<body>
      <form runat="server">
      <asp:Calendar id="cal1" runat="server" />
      Select the month to display: <br>
       <asp:DropDownList id="ddl1" runat="server">
           <asp:ListItem>1</asp:ListItem>
           <asp:ListItem>2</asp:ListItem>
           <asp:ListItem>3</asp:ListItem>
           <asp:ListItem>4</asp:ListItem>
           <asp:ListItem>5</asp:ListItem>
           <asp:ListItem>6</asp:ListItem>
           <asp:ListItem>7</asp:ListItem>
           <asp:ListItem>8</asp:ListItem>
           <asp:ListItem>9</asp:ListItem>
           <asp:ListItem>10</asp:ListItem>
           <asp:ListItem>11</asp:ListItem>
           <asp:ListItem>12</asp:ListItem>
       </asp:DropDownList><asp:Button id="bt1" Text="Submit" OnClick="ButtonClick" runat=server /></form>
</body>
</html>