Hi,
I would like to know how to display modal dialog after button click.
If you mean DayPilot.Modal (http://code.daypilot.org/81367/daypilot-modal) and a generic ASP.NET button you can try this:
<asp:Button id="Button1" runat="server" OnClientClick="new DayPilot.Modal().showUrl('Modal.aspx');" Text="Click me" />