Birthday presents would not be a birthday without birthday presents. With Fortnite's fifth birthday comes several quests that you can complete to earn experience points. One of these quests will involve you finding birthday presents and throwing them away in five different matches.
Fortnite Chapter 3, Season 4: How to Find Birthday Presents
Birthday Presents are generally fairly straightforward to locate, but they can also be quite difficult to obtain as a match progresses. In addition, you have a greater likelihood of finding them at the start of a match than near the middle or end because other players have likely already picked them up.
The birthday presents you're looking for are legendary items on the ground. Although listed as a legendary item, they aren't that uncommon to find, as if they were legendary or mythical weapons you'd loot off the bosses. From what we can tell, they can appear anywhere, but they have a higher probability of spawning while searching for it.
To receive the item, drop it onto the ground and complete the quest for the Fortnites Birthday celebration. You may choose to throw it on your feet or next to you, but look inside the present. Usually, there are a few help items waiting for you inside these packages, which will help you finish your match later on.