My friend said we have to walk to a gate and then it will say what swarm appears but when I did it,it just said the date and blah blah...what happened? Is there any other way?well I have not get the national dex and have not completed the game.do we have to do it first?
That's the reason. You need the National Pokedex and defeat the Elite Four.
After you complete the game, you will get told that random swarms will appear in different places so go to the gate and it will say what route it is on. Also as Mew said, you need the National pokedex which you get after completing the game.