From the WRTA website:
The WRTA will be installing a new farebox system the weekend of April 28 & 29, 2012. At this time, a new pass system will be implemented that will work with the new farebox system. Free fare will be offered on all fixed route buses on Saturday, April 28, 2012 and Sunday, April 29, 2012. WRTA ticket representatives will be at City Hall Plaza for the entire weekend to explain the changes in the passes and to help riders with ticket exchanges.