Your server-side event handling methods will get the important data:

EventClick knows the event value (PK column)

The argument of EventClick handler is set to the value of the clicked event (DataValueField).

private void DayPilotCalendar1_EventClick(string pk)
{
	Label1.Text = "Selected event: " + pk;
}

FreeTimeClick knows the time clicked

private void DayPilotCalendar1_FreeTimeClick(System.DateTime start)
{
	Label1.Text = "Selected time: " + start.ToString("s");
}