Drifloon is a very special Pokémon in Pokémon Brilliant Diamond and Shining Pearl, and you are only ever able to catch a single Drifloon on each save file. You have a single chance at catching it every week, but it will stop appearing all together after you catch the first one.
So how do you get Drifloon in Pokémon Brilliant Diamond and Shining Pearl?
You can find and catch Drifloon in Pokémon Brilliant Diamond and Shining Pearl by going to Valley Windworks on a Friday. It will appear right in front of the wind plant, and you can interact with it to start the fight. You must have fought off Team Galactic from the wind plant before it will spawn.
There is no method to get it on other days than Fridays, do you just have to wait if you missed it on the previous Friday.
It is possible to trick your Nintendo Switch to believe that it is in fact Friday by going to your console settings and changing the date. However, this will disable all event Pokémon from spawning for 24 hours. So change it to Thursday, wait 24 hours, and go find Drifloon in Valley Windworks like any Friday.
If the Drifloon faints when you fight it, you have to wait another week to be able to try again. But it will spawn every single week if you defeat it. Once you capture one, it will stop respawning completely, so you will only ever be able to get a single Drifloon on each save file.
It is possible to get multiple Drifloons by trading with other players.