I freely admit that this is for a trivia game at work, but I’ve been unable to find the answer (searching google using “password” as one of the search terms gives you sixty million sites that require people to enter a password).
A link with the answer or a suggestion on modifying the search terms would be just as welcome as the answer.
This sounds like it needs to be more specific. There were like dozens or even hundreds of passwords used by the air, sea, and ground force commanders to indicate all manner of things. Perhaps they are just asking for the name of the operation? The D-Day invasion was known as Overlord.
Like Dr. Lao says, there were hundreds of challenge and countersign pass words in use on The Day and on the following days. The challenge and counter sign (password) are recognition signals designed to tell if the guy you were confronting was likely enough to be a friend to let him get close to you so that you could go through more positive identification procedures. There was probably a different set used in each infantry battalion and on each radio network. The pass words themselves were derived from a table in the unit’s signals manual. The pass word changed each day so there were different pass words in a unit on different days. There were different pass words for different purposes. If you wanted to pass a neighboring unit’s lines there was one set of pass words; if you wanted to talk to a neighboring unit, or a superior or subordinate unit by radio or telephone there was a different set of challenges and countersigns. There is no one answer or simple answer to your question.