The biggest twist of Captain America: Civil War revolved around the history of the Marvel Cinematic Universe, and that Steve Rogers' best friend, Bucky Barnes, was actually the one that murdered Tony Stark's parents. He was being controlled at the time and wasn't in charge of his own actions, but still, he was the one physically responsible. Captain America kept this a secret from Tony to try and protect him, which wound up biting him later on down the line, inciting the war between the Avengers. So why are we talking about this today, more than three years after the release of Civil War? Well today is the anniversary of the death of Tony's parents, otherwise known as Mission Report Day.
Throughout the entire movie, Zemo (Daniel Bruhl) tries to get the information about that fateful mission from Bucky, aiming to bring down the Avengers in the process. He asks him on several occasions for "Mission report: December 16, 1991," hoping to get the details regarding the mission and the death of the Starks.
It takes Zemo several attempts to get through to Bucky, but he finally achieves his objective when Bucky is imprisoned and he poses as an employee of the United Nations in order to read all of his activation words. In Russian, Zemo reads the following code and once again turns Bucky into a killing machine:
"Longing. Rusted. Furnace. Daybreak. Benign. Nine. Homecoming. One. Freight car."
As you know, this turns everything upside-down. Steve continues to back Bucky as Tony seeks the truth. Once it's discovered, the two end up in a fight nearly to the death. The end up not speaking for years after that because of their differences. The next time the two heroes see one another, as far as we know, is after Thanos decimates half of the known universe.
Mission Report Day is a strange thing for Marvel fans to talk about, but it's such a particular date that it's basically burned into all of our minds. As soon as you see the 12/16 on a phone or calendar, it's hard not to say "Mission report" in your best Zemo voice.