Florence to Assisi

If you take the train, you can leave Florence as early as 06:40, connect in Terontola-Cortona, and arrive in Assisi at 09:35. On the return you can leave as late as 19:17 on a direct train to Florence which arrives at 21:48. For detailed timetables go to trenitalia.com or bahn.de. On the Trenitalia site enter a date within seven days from today to see the complete timetables that include regional trains.

Hi Laura, Tim's on the money with his Timetable advice. Just a warning, though, italian trains are notoriously late, so be aware of that when making connections. I got stuck in Cortona (new town, not old) for a few hours a couple of years ago on this very route (other direction). Keep that in mind when planning. BTW, you'll LOVE Assisi! It's my favorite Italian town. You'll probably regret not spending a night there, though, as it is magical in the evening.
Have fun!

Correction to my previous post. You can leave even earlier if you so choose. Depart at 05:50, connect in Terontola-Cortona, and arrive at 08:32.

Assisi is beautiful. The drawback with doing a daytrip is that thousands of others do the same thing each day, flooding the town from about 9 am until about 5 pm. It's best seen with at least an overnight, but you just don't have that option. The problem with the train option is that it drops you off in the lower town (called Santa Maria degli Angeli) and then you have to bus up into Assisi proper or take an obscenely expensive taxi. A direct bus from Florence to Assisi would be the way to go, imho, if you can find it. Have you asked your hotel/B&B in Florence if they know of such a bus? Here are some photos from our trip to Assisi in May 2008, in case you're interested. Happy travels.