In my experience using the free weekend-er coupons last year, different locations were blacked out on different dates. I think it depends on capacity controls at the locations. I have one coupon scheduled for this labor day (been booked for ~3 months) and the system took it. I tried another free weekend coupon for that weekend recently (duplicate booking, just to see if they blacked it out) and it was blacked out. So the trick might be booking early.

Twice last year, however, I wanted to use the coupon on blacked out dates, according to Avis.com. I called to make the reservation & was told it was blacked out. But the agent gave me the usual, "but I have a coupon here for $15 off your rental." I booked it and took the "free weekend coupon" to the counter when I picked up my car. The first time the agent said, "oh, that's not the coupon that's in the system. I wonder how that happened. Let me fix that and you'll be all set." I didn't say a word & let her "correct" the system. The second time, they never even said anything when I gave them the coupon. They typed on the computer for a minute and printed me a new contract. I thought I'd definitely have problems when I returned the car but both times they didn't charge me for the 2 free days. I can't guarantee that will work, but it's an option.