As of now, this year's National Championships will use the Standard format, which is XY and later sets. This is essentially what is currently playable in Standard, with the addition of any promo cards released between now and three weeks before the US Nationals (I am assuming you are asking about US Nationals because your icon says you are from Chicago). There is the remote possibility that TPCi will ban a card between now and then; remember that Lysandre's Trump Card was banned shortly before Nationals last year. However, these seems extremely unlikely. So... the Standard format of right now will presumably be played for Nationals.