Can you tell me what event you can get a Keldeo in?
You cannot catch a Keldeo in Pokemon XY.
But there are currently only 2 ways to get Keldeo:
Trade: Be it with a friend, acquaintance or passer-by, you can get a Keldeo if someone's willing to trade one to you.
Transfer from PokeBank: There have been a few events throughout the world during the past two years where Keldeo were being given. You can find a list here.
Unless there's another event where you can get Keldeo in Pokemon XY, those two above ways are the only ways to get Keldeo.