Visit Wrocław during the May holidays (April 30 - May 2) for three days of outdoor rock concerts at the Pergola next to Centennial Hall. Concerts begin each day at 15:00, heavy hitters of the Polish rock scene like Myslowitz, Nocny Strachy Na Lachy, Kult, Nosowska and Nocny Kochanek are all on the long bill, and Tuesday May 2'nd's final night will be headlined by Ralph Kaminski. Tickets are available online and can be purchased for 1, 2 or all 3 days. 
One other event is also taking place in connection with Wrocław's 3-Majówka celebrations. On Sunday May 1st at 10:00 on Wrocław's Market Square, in what has become an epic annual tradition, Vroclavers will once again attempt to break the Guinness World Record for the most people playing the same song on guitar at the same time. This year's tune is 'Hey Joe' by Jimi Hendrix, so grab your git and study up. More info about how to participate (it's free) here.