I would guess you have the event handler assigning code inside if (!IsPostBack) {} block:
if (!IsPostBack) {
// ...
testCalendar.FreeTimeClick += new DayPilot.Web.Ui.FreeClickDelegate
(testCalendar_FreeTimeClick);
// ...
}
You can try moving the code outside of that block or put it into the .aspx file:
<DayPilot:DayPilotCalendar ID="DayPilotCalendar1" runat="server"
...
OnEventClick="testCalendar_EventClick"
OnFreeTimeClick="testCalendar_FreeTimeClick">
</DayPilot:DayPilotCalendar>