Once you load your save file, or the save file of any town resident (other than creating a new player) the game sets in stone the random chances of the day. It will calculate time since the last game launch, determine which NPC's are visiting (Redd, Katrina, Sahara, etc), process any villager moves (in or out), and other things. Once this has occurred, there's no going back. This is by design as the game is very much about living with consequences and going with the flow.
I can provide you advice on how to avoid this in the future. There's something called the "new save file trick". If you're ever concerned about how your town will look you can start a new save file which does NOT save all the random variables to the cartridge until you save. Many people use this trick to control where villagers place their houses, but it has other uses. If you're unhappy with the random generated town status, just quit the game without saving and do a new save file (repeat until you have the desired status or villager location). If you are happy with the game then you can do one of two things:
You can quit without saving and reload a main save file. This
will cause a new random selection of town variables, but the date,
holiday/events, villager move outs will be the same.
You can save the new save file temporarily to lock the town
variables in place. Note that you'll need space to place a new
house before saving. If you don't want to keep the new save file,
just quit and delete it. Everything in your town will be just as it
was when you saved (except that the house will be gone and a dirt
patch will be in its place).
No matter what, time travelling is a tricky thing in Animal Crossing. Typically you should do it from the options within the game, and no more than 5 days at a time (this allows you an opportunity to save any villagers which might randomly try to leave).