Q: What criteria does iCal use to prioritize order of all day events?

Can someone please tell my what criteria iCal uses to determine the order of priority in which all day events are listed on a given day?

For example: let's say I have three 'all day' events that I need to enter on one day. I enter them in the order that I would like them to appear top to bottom, knowing that they will stack vertically on top of one another as three separate bars within that day.

However, as I enter these events (or after) they often re-stack their order, based on what? I don't know. It's not time-of-day because they are 'all day' events and it's also not alphabetical order. So what then?

I often have to trick iCal into presenting the events in the order I want them to appear by cutting and pasting the titles into the existing all day banners until I have the right combination. So I eventually get my calendar looking the way I want in iCal BUT...once I save the calendar as a PDF the order events are mixed up in the PDF! Whaaaaat?

I've been struggling for 10 years now to get iCal to simply let me enter events in the order I NEED them. What's the secret?