Here are the exact specifics about the program, which is an excellent benefit to Avis customers: (this came straight from the www.avis.com)

Renters who chose Avis time and again, earn a place in Avis First, our new program that rewards our best customers with a whole new level of rewards, including free weekends. This program is replacing Preferred Select.
Avis First members are the FIRST to enjoy access to high-end services and some of the finest vehicles in the rental car industry. Some of the benefits include:

Continued Preferred Service—skip the lines and go directly to your vehicle
Free* two-day weekend rentals earned with every third rental
Upgrades, when available—to get you to your destination in style
Dedicated 800 number available day and night—to service your needs
Members-only website that gives you access to your total rentals and total rewards earned.
* Certain terms and conditions apply
Member Qualifications

Membership is by invitation and a renter must have the following qualifications to be eligible:

You must be in Preferred Service.
In a calendar year, you must have 12 or more paid rentals or a cumulative rental day total of 35 or more days on one wizard number to qualify.