I saved, did the Gamble option, didn't win, returned to the main menu upon seeing the result (i.e. either the dialogue that says nothing happened, or the dialogue saying I won 10,000 Chollars), and then reloaded the save from the main menu. Another thing I'll note is that the sfx from the previous dialogue (from before returning to the main menu) tends to play again after initiating a new dialogue after reloading.
i.e. if I lost at gambling, when I reload and hit the Gamble option again, it'll play the sad losing sfx. And if I were to exit to the main menu during one of Nico's dialogue options, the sfx for Nico's text appearing on screen will play even if Nico isn't actually talking in the new scene. It only plays for a second, though, not for the full duration of the sfx.
The eternal day is 10/27, and I didn't save during dialogue, but instead on the Casino screen - I've since tried saving in other locations to see if that changed anything, but it didn't seem to affect the loop at all.
I DID return to an earlier save a while back, as I got Ending 6 along the way to Samson's ending, so I reloaded to before I cleaned the final area (my room), so I could return to my main goal after finishing my side goal. I completely exited out of the game between getting Ending 6 and going back to an earlier save, so I don't... THINK that's related? But it could be? Other than that, I mostly stick to one active save slot - though I cycle through which save slot that is (i.e. my first save is in slot 1, then my next save is in slot 2, and I keep going until I'm back at slot 1 again, at which point I cycle through all the slots again)
I've exited the game completely, and it hasn't fixed the problem with the day not advancing :( I think the save is just borked now.