Make an event recur based on "the last full weekend"

Question

I'd like to set up a recurring event so that I don't have to re-create one every year. However, the recurrence of the event is not something that can easily be handled by the calendar's built-in options. It's a full-weekend event, and occurs as late in the month as possible where the event still can be contained within that month.

For example, here's how the event would recur for months in 2011:

Month    Dates    Saturday        Sunday
Jan      29-30    5th (Last)      5th (Last)
Feb      26-27    4th (Last)      4th (Last)
Mar      26-27    4th (Last)      4th (Last)
Apr      23-24    4th             4th (Last)
May      28-29    4th (Last)      5th (Last)
Jun      25-26    4th (Last)      4th (Last)
Jul      30-31    5th (Last)      5th (Last)
Aug      27-28    4th (Last)      4th (Last)
Sep      24-25    4th (Last)      4th (Last)
Oct      29-30    5th (Last)      5th (Last)
Nov      26-27    4th (Last)      4th (Last)
Dec      24-25    4th             4th (Last)

As you can see, the event cannot reliably be triggered by:

• Any numeric date.
• Any 4th or 5th weekday.
• The last Saturday.

The only way I can think of to set this event to a single, recognizable trigger would be if I could set it to recur at the end of "the last Sunday" and have a duration of negative two days. Obviously, that's not going to be an option in most calendar programs.

Is there any other way I could configure this properly, that I haven't thought or become aware of?