The Zend Developer Zone has announced the bug hunt days for June happening the 23rd through the 25th:

For those who haven't put the reoccurring event in their calendar, this announcement is for you: the Zend Framework Monthly Bug-hunt is here again! Next Thursday, Friday and Saturday (the 23rd, 24th and 25th of June), we'll be hosting our monthly bug hunt. For those of you unfamiliar with the event, each month, we organize the community to help reduce the number of open issues reported against the framework.

If you're wanting to get involved but aren't sure where to start, you'll first need to get a CLA on file then you can start browsing unresolved issues of check out their "getting started" guide for more information.