An "editathon" is actually a Wikipedia concept that is not unique to this OFR collaboration. Wikipedia is crowd-edited by design, so an "editathon" is often held by experienced editors to help provide training to new editors, as well as to accomplish the basic purpose, to "edit and improve a specific topic or type of content."
Although this is a first-of-its-kind event for the National Archives, it is not the first time the Wikipedia community has hooked up with federal employees. In May last year when furloughs were hitting some government workers, the D.C. chapter of Wikimedia held a "Furloughed Feds Workshop", a "training seminar and edit-a-thon for federal employees interested in learning more on how to edit Wikipedia."
http://www.weeklystandard.com/blogs/feds-collaborate-wikipedia-editors-improve-wikipedia-entries-related-government-publications_794261.html