Have you heard of Driffloon but never found one to catch? This is a special Pokemon that can only be in one place, once a week, so you need patience. Read on to catch him.
Steps

Step 1. Wait for Friday
You can change your DS clock to Friday if you don't want to wait too long.

Step 2. Travel to the east of Floroma city
This is where Valley Windworks is located, where one of the game's events is usually held.

Step 3. Look for the Drifmoon in front of the Windworks
You will see him in the form of an overground ghost, as some of the legendary Pokémon of the game appear, and not in the grass.

Step 4. Talk to him and catch him
Congratulations on the addition of the team!
Warnings
- You will not be able to catch a Pokemon if you bring it to a swoon. Don't use attacks that are too powerful against Driflun.
- Directly changing the DS clock to Friday will not spawn Drifloon. (once you set the date to Friday, enter the building, then exit and the Drifloon will appear) (checked 3/15/2011)