This year the Bonanza, Cessna and Mooney mass fly-ins were on Saturday, two days prior to the start of Airventure. That may have been the agreed arrangement with the FAA, or was just the preference of the group organizers. I would guess the FAA ATC prefers Saturday; Sunday would be a little too busy to shut down arrivals at RIPON for a group arrival. Basically when the group calls inbound a few minutes out, the tower lets a few more regular inbounds land, then clears the entire group to land. The tower is then silent until the last of the group lands. The mass groups do not follow the RIPON/FISKE route. This allows the entire group to land, taxi and park in one area and setup camp as a group. If a few planes fly in together in sequence they'll get separated by other random aircraft on the Notam arrival, land on two different runways, and once on the ground get directed to different parking areas. Holding up your GAC (general aviation camping) sign could get you parked in a different area than your friend that split off and landed on the other runway.
As Paul and Bob S. mentioned above, there are required sign-offs and waivers involved. I don't know how much preparation time that requires, but I believe the Bonanza/Baron formation practice happens throughout the year, not just one day prior to OSH.
This year my sons and I went two days early just to watch the arrivals. It was most entertaining. By coincidence, we stayed in RFD, Illinois Friday night. On short final we noticed a strange array of lights on the parallel taxiway. Turned out to be 40 Bonanzas and Barons waiting to depart together and practice, still working on endorsements. There were 100 more parked on the ramp. Their arrival at OSH was impressive. They did a flyover and break in groups of 15 or so, then landed in tight groups of 3, two on 36L and one on 36R. The Cessna and Mooney groups started with groups of 3 landing at once, but started to separate and get staggered, just not as sharp.