Pro Football Talk has acquired the offseason schedules of each NFL team for 2013 and gathered them in one location. I'll save you the time and trouble of clicking on the link though, and let you know what you came here to see.
The Green Bay Packers will begin their offseason program on April 15th, and must close their doors on June 14th. Towards the end of the open dates is the mandatory minicamp, which will take place from June 4th through the 6th.
Mike McCarthy's quarterback school will presumably take place in the early part of this schedule, likely before the draft. He expressed displeasure last year in the new NFL offseason, but will likely devote some time in the first few weeks to working with the Packers' quarterbacks. This will likely be very beneficial for B.J. Coleman, who will be going through the program for the first time.
The 15th will also likely be the first time that the Packers get to see returning defensive lineman Johnny Jolly back on the field. Hopefully he will show up in shape and ready to devote his time to becoming a productive NFL player once again.
Most other teams across the league seem to schedule the minicamps at the very end of their offseason program. The fact that the Packers have an additional week afterwards is a bit of an oddity, but likely a result of McCarthy's preferences.