You can attend qualifying shows right up to the first day of nationals - however, it would be best to get an attendance verification from the show manager for any last minute shows - especially if your horse does not place in any class. The Master List, sent in by the show manager, tells the office which horses attended that show - even if they did not place. Once in a GREAT while, a horse gets left off the Master List. If that were to be your horse, and he did place at Nationals - his win may not count if one of his qualifying shows didn't show him on the Master List. The Show dept would contact you (they may not have in the past, but they will this year) and inform you that your horses qualifying shows for Nationals were not met - giving you a chance to produce proof that they were. Many Show Managers have a simple form to give you - "so and so was shown at such and such show on this date "and then signed by the Show Manager - just as a back up to the Master List.