We'd like to visit Assisi the 2nd week in September of this year (2014). Best day of the week to do so? We'll take a day tour from Rome.
Weekend was extremely crowded the last two years (early June).
Rick's guide is spot on-the city empties by 3 as the tours leave-we've only been ther during the week, not on the weekends, however, after 3 it's pretty empty.
I'm in Assisi for nearly 3 weeks every year (international music festival in July). Saturdays are very crowded with tourists and young people high on the spritie of St. Francis. During the week the tour buses arrive in the morning and leave fairly early in the afternoon. It's painful to watch these people being herded through the ancient city so quickly! Assisi is a place to wander and to wonder. The big sites are truly important to see. But don't avoid a staircase or alley that seems to go off the beaten path. There's magic in those places too. Go slowly, investigate, stop for a gelato or some wonderful Umbrian wine. Look at the local artists galleries - I've bought lots of pieces over the years that bring a smile to my face and my heart. Buy leather goods! My favorite, Maurro, has the most amazing sandals, bags, book covers, belts - you name it! Assisi at night is a totally different experience. It calms down, and encourages you to do the same. And the views of the valley!!! Oh my. Don't spend just a day, stay over a night or two if you possibly can! I'm coming up on year 13 in Assisi. I start counting the days the day after they put me kicking and screaming on the plane home from the previous year.
I think these are good responses.
Yes, I know now to avoid the weekends and to explore and savor the sights that are off the beaten track.