Ajax AsyncPostBackTrigger

<table>
<tr>
<td>
<asp:UpdatePanel ID="Updmonth" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlMonth" runat="server" Width="110px" AppendDataBoundItems="True"
AutoPostBack="True" OnSelectedIndexChanged="ddlMonth_SelectedIndexChanged">
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</td>
<td>
<asp:UpdatePanel ID="upddate" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlDate" runat="server" Width="110px" AppendDataBoundItems="True">
</asp:DropDownList>
</ContentTemplate>
<Triggers>

<asp:AsyncPostBackTrigger ControlID="ddlMonth" EventName="SelectedIndexChanged" />

</Triggers>

</asp:UpdatePanel>
</td>
<td>
<asp:DropDownList ID="ddlYear" runat="server" Width="110px" AppendDataBoundItems="True">
</asp:DropDownList>
</td>
</tr>
</table>