I'm also staying in Z5 but for 2 nights so the mobilis weekly at 22 euros seems the best option (I think!)
If it is the Navigo Weekly ticket you have seen for 22.80 zones 1-5, they run from Monday to Sunday, and the last day they can be purchased for this week (ie Final weekend) is the Thursday.
below is off their website
Navigo Pass DurationA Navigo Week Pass is valid from Monday at 00:00 to Sunday 23:59:59.
You can buy Navigo cards / passes for the current week up until Thursday, midnight. Next week’s Navigo card is on sale from Friday morning onwards. (In the past, the Navigo weekly train pass for the current week stopped selling at Wednesday midnight, but this has been moved to Thursday midnight.)
Note that you cannot choose the start & end date of your weekly Navigo pass; it is strictly Monday morning to Sunday midnight, regardless of what day of the week you buy the pass