u need to add ajax dll to u r application.
then register it in your page where u want to use..
User ajax calender extender its very easy....
like this
<asp:textbox id="txtDate" runat="server" cssclass="textbox" />
<asp:imagebutton id="imgcalnd" runat="server" height="16" width="16" imageurl="~/images/Calendar.png" />
<cc1:calendarextender id="calcontl" runat="server" targetcontrolid="txtDate" Format="MM" popupbuttonid="imgcalnd" />
to get only month use this property in extender
Format="MM"