Apparently the alternate name for this day is Maybe Day, as in RAW's "maybe logic." That's the preferred name for today from Wilson scholar Eric Wagner.
In 2003, however, July 23 was proclaimed "Robert Anton Wilson Day" for the city of Santa Cruz, Calif.
The proclamation was issued July 22, 2003, by Santa Cruz mayor Emily Reilly. The full text of the mayor's proclamation is here.