To take part in an enjoyable, exciting occasion or pursuit. The expression dates from the mid-1700s and originally meant a special day set aside for troop maneuvers and exercises, as it still does in military circles. Early in the 1800s it began to be transferred to civilian occasions, at first involving groups of people (such as a school outing), and later to any pleasant experience, as in “Mike’s having a field day with his new camera.”