Greetings Jerome,
My idea was not a package with a cycle of 8 or 9 weeks. Rather it was a large number of (one-time) packages, with no cycles, only offsets at incrementing intervals, which you'd need to align with the plan schedule start date. Perhaps a worked example will help:
1. Setup a strategy with incrementing offsets, for example as such:
2. Assign ALL packages to the Operations in the Task List that you want performed each time.
3. Schedule the plan from a specific date (03.01.2014 in this example)
Note that this strategy is very bespoke, specific and tailored; it assumes a certain start date on a certain year; also, there are some limitations here, this strategy isn't self sustainable indefinitely in the long term as the cycle would need to effectively be 28 years (1456 weeks!) for that to repeat itself over with respect to the third Friday of every second month 
Realistically, you can look at setting up this strategy for 16 years (96 packages), at which point the plans will have to be re-started.
So on the downside, you would need specific strategies for various cycles, offsets and start dates (though you can also fine-tune the schedule on plan level in IP10)
On the upside, once you have the strategy in place, you can use it to schedule more than one Maintenance Plan - assuming the maintenance indeed always falls on the third of the month.
Hope this example helps you!