A field day is a special school event where students spend the day outside playing games, competing in races, and enjoying activities like relay races, tug-of-war, water balloon tosses, and obstacle courses. Instead of sitting at desks doing regular schoolwork, everyone gathers on the playground or athletic field for a celebration of fun, teamwork, and friendly competition. Many schools hold a field day near the end of the year as a reward for students' hard work.

The phrase also means having a wonderful time doing something you love, especially when you get an unexpected opportunity. If your teacher accidentally writes a grammatically incorrect sentence on the board, the class grammar expert might have a field day pointing it out. When journalists discover a major scandal, people say the media is having a field day with the story. A talented soccer player might have a field day when playing against a weak defensive team, scoring goal after goal.

Today it suggests either organized outdoor fun or the joy of finding yourself in your element, with plenty of opportunities to shine.