As Ereo asked, does this happen with every save file or just one particular save? If you only have the one game save file so far, you might want to try creating a new save file to see if it happens again.
What happens if you get out of bed on the 26th and immediately go back to bed to trigger a save? In other words, get out of bed, don't do any activity for that day, and immediately go back to bed to save.
Do you have iCloud enabled and the Files app on your phone? If you do, you can back up your save file(s) temporarily to iCloud:
- go to the Files app > On My iPhone and you'll see the Stardew Valley folder.
- Open the SDV folder, select the save file folder. It's named in the format of your farmer name___some numbers, i.e., abc_12345678.
- Press and hold that save file folder and select Move from the menu. Even though it says move, it will actually just copy it to iCloud. This will save your data.
- Back on your home screen, press and hold the Stardew Valley app and delete it.
- Reinstall SDV. To create the game folder, create a new save file. You don't have to do anything in that save file except set up the basic options, let it launch, and then use the option to Exit to Title. You can delete that temporary save file later. This step just creates the folder in On My iPhone.
- Go back to the Files app > iCloud and move the save file folder back to the SDV folder in On My iPhone.
- Launch the game, choose your save file farm, and see what happens on Summer 26.
