In your save (on Windows: %appdata%/StardewValley/Saves/<name_seed>), there's two files, one with the same name as your farm or farmer, followed by a number. That's your save for the day. There's another one, same name, ended by _old. That's the save of the previous in-game day. Rename that one, deleting the _old from the name, and you should be able to basically make the game forget the last ingame day.
Make a copy of the whole folder somewhere safe, before starting editing any files.