I had actually not known there was a National testing day but it makes sense, especially if you live by the mantra of going every 6 months. So here you go, mid year/late June and then not again until January/December.
Go out and get it done. Your doctor or free clinic; get it done.
If your are in NYC I can tell you that every Friday the gay drugstore, next to Spice, on 20th and 8th gives free HIV screenings.