https://www.gamespot.com/articles/fortnite-1st-birthday-challenge-guide-dance-in-fro/1100-6460687/

To celebrate the first birthday of Fortnite, developer Epic Games has introduced a special set of challenges to the game as part of the 5.10 update. There are a total of three birthday challenges, and if you're able to complete all of them you'll get a special reward item. Thankfully, each of the challenges are fairly easy, though one of them will require spending a little bit of time running around the map and getting your dance on.

The first of the challenges involves playing 14 matches in the Battle Royale mode, which you can probably manage with ease. The second involves being a little aggressive in-game and dealing damage to opponents. You'll have to dish out a total of 1000 points of hurt before you're rewarded with the birthday emote. The third of the challenges requires you to dance in front of 10 different birthday cakes, and once you've done that you'll get a spray.