Details about various NPARC “Field Days” - Future, Present & Past - can be found at the following links:  2010, 2009, 2008, 2007, 2006, 2004

Field Day is held annually on the fourth full weekend in June.

Instead of making up our own definition, we'll quote a workable (american) one from a relatively reliable web site.

Field Day is an annual amateur radio exercise, widely sponsored by IARU regions and member organizations, encouraging emergency communications preparedness among amateur radio operators. In the United States, it is typically the largest single emergency preparedness exercise in the country, with over 30,000 operators participating each year.