I just purchased regional tickets from S. Minitao-Fucecchio to Firenze S.M. Novella. These tickets have a bar code and a button to check-in (iPhone app).
My other tickets, which operate by Frecciarossa from Venezia S. Luica to Firenze S.M. Novella don't require a check-in.
My question is when should I check-in? The day of, when the train arrives, day before, etc.
with a regional ticket you can check in anytime after 12:01am on the travel day the ticket is for.
If this is a standalone journey, which it sounds like it is, then check in anytime that day that is convenient. Although once you've checked in then you can't change the ticket to take an earlier train if that might be a consideration.
If you were arriving at S. Minitao-Fucecchio on a different train then it might make sense to wait until you have almost arrived (in case it's late, and assuming you have internet access) since you could easily change the regional train you've chosen before checking in. See https://www.trenitalia.com/en/purchase/digital-regional-ticket.html But according to the thread https://community.ricksteves.com/travel-forum/italy/question-new-regionale-digital-ticket-check-in-and-missing-tight-connections-on-one-ticket-route even if you've checked in there is still a 4-hour window after your scheduled train for which the ticket is valid.
Are you using the official Trenitalia app or did you purchase the ticket from a different vendor?
I have never encountered a check-in with Trenitalia BUT -- it has been a couple of years. So this could a new way to validate a ticket. In the past the regional ticket would have required a time stamp to open a valid four hour window to use. The other tickets don't have it because that ticket is only good for that day and time. It only valid for one train at a specific time. My guess is you check in about one minute prior to boarding. Regionale tickets have historically had a four hour time window when they are valid.
Your ticket is validated by checking in.
If you are a registered passenger, you can check-in in the “My journeys” area on this website or on the Trenitalia App.
If you are not registered, you can check-in using the function available in the e-mail or text message you received when you bought your ticket or on the Trenitalia App if you added and saved your Digital Regional Ticket in the “My Journeys-Saved Tickets” area.
Check-in is available starting at midnight on the day before your journey until the scheduled departure time of the train you selected upon purchase: the following message is displayed on the key: “Check-in and travel”.
Once you have checked in, the message “validated” appears with a dynamic green bar to confirm that your ticket is validated.
The Digital Regional Ticket is only valid for the train(s) for which you have completed check-in.
My guess is you check in about one minute prior to boarding
The earlier post is correct - you can validate starting at 12:01 AM on the day of the Regionale journey.
However, once you validate, the ticket cannot be changed, so better to wait until at least the morning of the journey.
Your four hour window starts with the departure time of the Regionale train. In the case of a connection, if the arriving train was late, you would have four hours to take a later Regionale train if you missed the scheduled one.
There are some concerns regarding connectivity and validation - if you don't have cell/data service, is best to validate while connected to wi-fi in the morning.
Once the ticket is validated, a green bar will be displayed.
I had this same question before my trip. I just came back after 12 days. We traveled on Frecciarossa on three separate trips but only on one of the trips it allowed me to "check-in" from the ticket sent to my email. I'm not sure why one trip allowed me to "check-in" and the other two didn't (the check-in button was grayed out). The train official did come around and asked for my ticket and there was no issue with the trips that I wasn't able to "check-in". And I did upload my tickets onto the Trenitalia app but there was no check-in button available. I bought the tickets directly on Trenitalia online (not through apps). You might encounter the same issue.
We traveled on Frecciarossa on three separate trips...
All seats on Frecciarossa trains are reserved; you are assigned a specific train, departure time, carriage and seat #. These tickets don't require checking in or validation. Regionale trains don't have seat or carriage assignments, and tickets do have to either be physically validated (paper tickets) or you have to check in per John, Jean and Markcw's posts. Also:
https://www.seat61.com/train-travel-in-italy.htm#regionale
