I'm not sure who makes the final determination for the official ice-out date every year but it is an annual event that helps to mark the passage from winter to spring. Maine's Department of Conservation says that ice-out is "...to be when you can navigate unimpeded from one end of the water body to the other. There may still be ice in coves or along the shoreline but when a person can traverse the entire water body without being stopped by ice flows, we will consider the ice to be out." Records have been kept for many years and you quickly see that the dates jump around quite a bit from year-to-year. Follow the link to Kezar Lake's records on this United States Geological Survey page.
Ice-out also is used for fund-raising purposes. A local favorite, Frosty, is on Moose Pond in Bridgton and can be seen from the Route 302 causeway. New this year is the Town of Naples Causeway Restoration Committee. For both of these contests, you must select the correct date to win 1/2 of the proceeds that are collected. I've also heard of restaurants providing the winner with a free meal and I'm sure there are other fun contests out there!