Hobbies And Interests
Locating a Swarm
The location of a Pokemon swarm is chosen at random every day. Tune in to Professor Oak's Pokemon Talk radio show with the radio on your Pokegear every day to learn where that day's swarm is located. The location of the swarm is also marked with an exclamation mark on the map. Travel to the specified location and run around until a wild Pokemon of the swarming species appears, then catch it as you normally would.
Before Obtaining the National Pokedex
Before you complete the Johto Pokedex, only Pokemon listed in it can appear in swarms. The possible swarms are Dunsparce in the Dark Cave, Snubbull in Route 38, Ditto in Route 47, Marill in Mt. Mortar, Remoraid in Route 44, Yanma in Route 35 and Qwilfish in Route 32.
After Obtaining the National Pokedex
After you have completed the Johto Pokedex and obtained the National Pokedex, Pokemon not included in the Johto Pokedex start appearing in swarms in addition to those that appeared before. The additional possible swarms are Whiscash in Violet City, Kriketot in Viridian Forest, Luvdisc in Route 27, Ralts in Route 34, Wingull in Vermillion City, Poochyena in Route 1, Chansey in Route 13, Clamperl in Route 19, Buneary in Route 25, Relicanth in Route 12, Swablu in Route 45, Mawile in Route 9 and Gulpin in Route 3. The latter two swarms are exclusive to "Pokemon SoulSilver," and are replaced respectively by Sableye and Baltoy in "Pokemon HeartGold."
Considerations
If you want to complete both the Johto and the National Pokedex, check which Pokemon swarm is active every day, since swarms are the only way to obtain certain species of Pokemon. If, on the other hand, you only want to move through the game's plot and don't care about Pokedex completion, you can safely ignore swarms, unless you want a specific swarm-only Pokemon for other reasons such as aesthetics or battle strength.