If you aren't able to buy them through the post in advance (as per Anemone's post above) then if you want the 20% off family discount for buying 5 APs at the same time, for members of your immediate family, you'll have to buy them at the park gates. This means if you buy them on a Thursday, you can use them that day, but then Friday and Saturday are blocked out. You will all have to be there on Thursday, with all your photo ID (ie all your passports) so the APs can be made up for you at the AP office inside the park.