If you log-in to Avisfirst.com and ensure you're on the "Home" page tab, there's the following text:

"We start you off with a free weekend rental

Then, after every third rental of 2 or more days you make with us, we'll give you another 2-day weekend rental on us up to 8 times per year. Learn more about the special offers you 'll receive as an Avis First member. It's not often that a rental car company puts you first. But that's the kind of treatment to expect from your Avis First membership."

Kind of disappointing to find out as my rental this week was only 19 hours, but that's a rarity for me...