It will be busy no matter when you go, and can be a bit overwhelming when you see the size of the whole operation.
When you go should depend on if there is something special that you want to see that is only going to be on a certain day. There are night time airshows using special lighting, and pyrotechnics on the planes, which is fun to see if you haven't before. In the past those have been on Wed, and Sat nights
There are a couple privately owned camping areas not run by the EAA, so you don't have to be a member at those, but they don't advertise, and don't take reservations. Relleumtdats (Stadtmueller spelled backwards) is the one I usually use when I don't stay on the EAA grounds. It can sometimes fill up early. It only has scattered banks of portapotties for facilities, but with the CI that's probably all you need. It is south of the main Camp Sholler EAA campgrounds near the ultralight runway, and the sites are closer to the action than many of the EAA sites other than the premium ones that have been taken and paid for a month before the show starts, just to get the few tree shaded close in sites.
Relleumtdats have no minimum stay (unlike the EAA 3 day minimum), and the last time I stayed it was $20/night. I thinks the EAA sites are $40+, and they charge you for all the nights remaining in the show, then refund any unused nights when you leave if you stop at the gate and ask, except for the three day minimum. The EAA grounds also have banks of portapotties, but do have fairly nice shower houses scattered around the grounds as well as some flush toilets here and there too, a camp grocery store, and is generally better organized, so there are some advantages to staying there.
Many people start coming during the week before the show starts to get better camping spots, or to watch as all the planes come in over the weekend before the show starts. It is first come first serve for most sites in both campgrounds. I think they do take reservations for the few EAA sites that have hookups, but you start getting charged for them from the day you reserve them, and I think they open the reservations a month before the show, so it gets very expensive for those.
There does seems to be kind of a mid week turnover from Wed-Fri where some folks who came during the week before the show start to leave, which often opens up some nicer closer in sites, but it would just be luck if you happen to be there as they pull out of their site, and take it over before someone else.
The afternoon airshow usually has some of the same performers each day, but it also usually has a theme for that day that will feature something special to differentiate it from the day before or after. The EAA Airventure website will have a calendar of events for each day describing each days airshow theme, and performers, and any other special events on the grounds for each day, so take a look at that to see if there is anything in particular that peeks your interest, and plan your schedule around that. There are also hundreds of workshops each day, so if there are any areas of aviation you are interested in learning more about, review the lecture and workshops calendar on the website to see what is being offered each day. There are things like welding, composite or fabric aircraft construction, etc Many of those are repeated on several days, but not all of them.
There is also the EAA seaplane base several miles away that you can take a shuttle bus to, and I have found it to be worth the time and cost to go to if you are interested in seaplanes.
As far as tickets go, you should decide what you want to see, or worshops you want to attend, and purchase tickets for those days. If you plan on multiple days, or even the whole week, there are packages where you get a little break on the price for multiple days...you'll just have to decide how many days, and how much money you want to spend.
Hope that gives you a little insight. Cary is also a lifetime EAA member, attends AirVenture each year, and may have some additional insight to share as well.
