To complete the Pokedex in Pokemon Brilliant Diamond & Shining Pearl (BDSP), you will need to collect a Drifloon. There is only one Drifloon in the entire game at it appears only on a specific day and time at a specific location. So, it might be a little hard to find it. This guide shows the location of Dffloon and the day and time when you can find it.
Also Read: How to get all Gift Pokemons
Where to Find Drifloon (Balloon Pokemon)
You will find Drifloon in Valley Windworks (Wind Farm), which is on the east side of Floaroma Town at the location shown in the picture below. Drifloon will be available at the highlighted location only on Fridays from 10 AM to 8 PM. Drifloon will appear only after you have progressed further into the game till you have cleared Team Galactic out of the Valley Windworks. If you chased out Team Galactic on a Friday, then the Drifloon won’t spawn until the next Friday.
If you aren’t able to catch Drifloon, then it will appear again the next Friday. If you catch it, then another one won’t spawn as there is only one Drifloon in the entire game.
You can change the Date and Time in the System Settings of your Nintendo Switch to make the Drifloon appear.