When to buy train tickets to Venice from Florence

You can book them now on the Trenitalia website. The super economy fares seem to be available on the trains I looked at. Just be aware they are nonrefundable, and non exchangeable. For travel tips for trains I have 2 suggestions. On this site, look under Travel Tips, then Transportation, for Rick's trips on riding the rails. The best website around for everything to do with European trains is the Man in Seat 61 website.

Also look at Italotreno, just as fast, often a little cheaper and trains are nicer (say some people).
https://www.italotreno.it/en
Prices only go up, so the sooner you can commit to a nonrefundable ticket, the better the price.

We buy them as soon as our opening day allows - we go 1st class as it's so cheap on opening day. The earlier you buy them is usually the best price.

Thanks for the tip! I didn't realize there were 2 companies that operated the trains.
The man in seat 61 had some great info. I did a little more online reading and found this site, which explained quite a bit about the trains and the booking sites.
https://www.tiltedmap.com/italy-train-travel-guide-italiarail-trenitalia-italo/
Seems I can buy tickets pretty far in advance and not worry about them selling out or prices rising.
Very helpful, thanks everyone!

Whichever trains lines you are using, don't forget the associated phone apps. Just like the airline apps, you can get your boarding passes, book tickets, etc. Both Trenitalia and Italo apps worked great for my last September.