Field Day is an annual Amateur Radio exercise, dating back to 1933, which is an educational event, an operating event, and a public relations event. This event is held on the 4th weekend in June; in 2016 that is June 25th and 26th.
Field Day gives Amateur Radio (AKA Ham Radio) operators an opportunity to learn about communicating in abnormal situations under less than optimal conditions. Hams practice these these communication skills by setting up temporary stations, often in public locations, and attempt to contact as many other Amateur Radio stations as possible. These temporary stations frequently utilize field expdient antennas and off-the-grid power such as solar or generators.