At Route 228 you find beldum only in swarm so what mean this and when its start I need to know this answer because 25% of my favorite Pokemons are finable only in swarm,so I wait the most good answer.
You to have the National Pokédex. Once you do, go see your rival's sister in Sandgem Town. She will tell you where the swarm is and what Pokémon is there. Once known, you can go there and find out. The swarms change every day so always check so that you can get the Pokémon you want