Having thought about it, I don't know if a National Day of Mourning is really appropriate either. It's too small. What should really happen is that the EU should declare Friday an EU-wide Day of Mourning.
Eight of the 25 (and part of another, former E. Germany) member states owe their current status as EU members to John Paul II more than to any other person. I haven't seen any official EU reaction, but failure to acknowledge John Paul II's role in the growth of the EU would be plain wrong.
EU Commission President José Manuel Barroso admitted that John Paul II deserved "the title of the founding father of a united Europe", which is exactly what he is. A National Day of Mourning is too small for John Paul II. He deserves a Supranational Day of Mourning (still without a public holiday, please).