IDT include file are simple text files that contain a date and some text. iCal will will read the date and place the text for that date onto your calendar. iCal will do this if:
IDT files are text files that end with a .idt extension. These files conform to the old Window INI format. Here is a very simple idt file.
01/01/2005 Beth's Birthday
02/04/2005 Tim's Birthday
10/31/2005 Mary's Birthday
12/22/2005 Davids' Birthday
This is the simplest and most common way to create the idt file. The Description is what you will see Administration General Settings calendar page. The date must have the format MM/DD/YYYY. The dates must be in chronological order. There can be any number of spaces or tabs before the start of the calendar text.
You can use keyword assignment for the text. The moon phase idt that ships with iCal demonstates this.
Finally you should note that the text can be html. Here is an example that would include graphical moon images onto the calendar.
Description=Graphical Moon Phases
01/05/2005 <img src="/fullmoon.gif">Full
01/13/2005 <img src="/q3moon.gif">Q3
01/20/2005 <img src="/newmoon.gif">New
01/27/2005 <img src="/q1moon.gif">Q1