The Fancy Formal decal was added to Rocket League In July of 2020. It was listed as a very rare item, and was added as part of the celebration for the fifth anniversary of Rocket League. It was one of the featured items of the week on the in game item shop. Fancy Formal is also one of a handful of decals that doesn't change color or design based on which team the player is on. No matter what, the decal always stayed black and white.
Fancy Formal Rocket League: How to Get
Unfortunately, there is no current way to acquire the Fancy Formal decal. The item was listed as very rare rather than a limited time item. Despite this, the item has not been purchasable since it was last available in the shop back in July. On top of that, the featured item was listed as "Not Tradable." This meant that players couldn't purchase the item and then trade it away. For players, this meant that the only time they could get this decal was by purchasing it that week in July.