Piece of cake.
🎒 Birthday Cake backpack is a Rare Fortnite BackBling.
📅 This backbling was added at Fortnite Battle Royale on 3 April 2019 (Chapter 1 Season 5 Patch 8.20-CU).
The back bling Birthday Cake belongs to Chapter 1 Season 5. It's not yet known how to get the cosmetic Birthday Cake.