The Longest Day is the name of a 1980 board wargame designed by Randall Reed and published by Avalon Hill. It simulates the D-Day invasion, Allied build-up and subsequent Normandy breakout, with scenarios for the attack on Cherbourg, the Cobra Offensive, the Mortain Counterattack and the Falaise Pocket.
It is a "monster game" meaning the map is very large, and there are 1,600 game pieces. Setup takes 8 hours or more, while the full game can take over 100 hours to complete.
The game is known for its historical detail--for example, the Germans forces are represented by several hundred, often unique pieces representing the historical units. Unlike most wargames of the era, the counters use not 1970s NATO symbols (cross, oval, blob and slash for infantry, armour, artillery and cavalry) but rather the symbols used on German World War II situation maps.