Basically, loading up your town (like when Isabelle is loading your town) also saves the game, so if say, on the 8th of july you loaded up your mayor at all, the day would be saved, so everything (the campsite villager, number of flowers spawned, weed numbers, etc etc) would be set for the day. so on july 8th if someone was moving in and you loaded up your mayor, wherever they put their house is pretty much set and you can't change it
plot resetting works because when you create a new character, until you place your tent down or whatever, the game can't be saved. so if you turned your ds off before you placed your house down and got your town card thing and everything from isabelle, the day wouldn't be saved/set
to plot reset,
1. on a day where you're expecting someone to place down the plot for their house (the fence thingy) DO NOT load up any of your characters
-if you need to TT to their moving date, you'll have to change the 3DS' clock in the settings menu (on the home menu) rather than the game's clock to do that. Changing the games clock would require loading the town!
2. create a new character, name it whatever you want, go through the train journey and all that stuff
3. Once you reach the town, run round and look for a house plot-fence-thing
4. If you don't like the location of the house plot, turn the 3ds off/return to the home menu of the 3ds, and start again at step 2. everytime you create a new character in this way the house plot should be randomized
5. if you DO like the placement of the house, then continue the begginer sequence thing where you go to isabelle and she tells you to go to nook and you get a house and then talk to isabelle again and everything... at the end of it the game will save and you'll finally be able to save on that character
6. thats it! you can just delete the character now if you want. if you load up the game on any other character the villager's house plot will now be int he place you picked
warning: it's very tedious!!
Edit: Also I forgot to mention... the days on acnl start at 6am... it's handy to know