Instructions
Speak with your Other Rival's sister in Sandgem Town. She will tell you which Pokemon is swarming. If it is not the one you want, turn your Nintendo DS off and remove the game cartridge.
Turn the Nintendo DS back on, and it will launch to the home screen.
Touch the "Settings" button --- it looks like a small Nintendo DS or a wrench, depending on which version of the DS you own.
Tap the calendar icon, then the clock icon on the next screen.
Set the time to 11:58 p.m. and save your settings. Turn the Nintendo DS off after saving.
Insert your Pokemon game and turn the DS back on. Load your saved file and wait a couple of minutes. When the time rolls over from 12:00 a.m. to 12:01 a.m., the old swarm will expire and a new one will load.
Speak with your Other Rival's sister again to find out which swarm has been chosen for the day.