Formal: 76 is Soldier: 76's Overwatch Storm Rising skin. Storm Rising is the 2019 Overwatch Archive event.
Overwatch Archive events allow players to experience key missions in Overwatch's past. It's one of the few ways for players to learn more about the lore.
Formal: 76 Soldier: 76 Overwatch Skin
So far, McCree and Formal: 76 seem to have the most unique cosmetic change.
The event seems to take place some time after the King’s Row mission. Reinhardt recently had something happen to him so he is sidelined. The mission will have Tracer leading with Mercy, Genji, and Winston as support. It looks like you will be hunting down Doomfist or his shell company in Havana, Cuba.
The Overwatch event will be made available on April 16 through May 6.
Photo courtesy of Blizzard