I think a little more clarification may be in order.
When you go online to purchase your improved water/electric site, you will see a map that shows each individual, numbered site. As a site is purchased, it will immediately change color, so you can easily see which sites have been purchased and which are still available. This constant updating allows you to monitor which sites in which locations are still available, so that you may delay before purchasing your site, if you wish. There is no need to purchase your site on opening day (July 2) if all you want is to get a site and you won't arrive until later - you can wait it out until the number of remaining sites gets close to selling out, then pull the trigger to get one before they are gone. Be aware that in the past, the last site was sold about 1 week prior to opening day, so paying for 2 weeks minimum was common. This year, it may or may not work out that way, so monitor the website closely if you wait to purchase your site.