John Mark Ockerbloom writes: “Many of us are celebrating a new public domain day. And we’re not just celebrating famous characters like Mickey Mouse, but all kinds of cultural works and information resources. A lot of later works are (not so obviously) in the public domain. At the Libraries at the University of Pennsylvania in Philadelphia, we started what we call the Deep Backfile serials project. We wrote some code to identify serials we held that might be in the public domain. For many serials, copyrights were rarely renewed in their early days, so a number of issues from the 1960s can freely go online.