If you choose to attend both the Welcome Dinner and Wedding Celebration you only need to be in Florence September 4th-6th (Though it is an amazing city so you may want more time) We encourage you to hop on a train before or after the wedding and explore Italy, make the most of your trip!
Be sure and contact your financial institution to put a "Travel Notice" on your account while you are away. The American Dollar to Euro exchange rates in Europe tend to be lower than what you will find in most American Airports, we recommend waiting until you have arrived to exchange any money. You may choose to use the ATM at the airport when you arrive. It is also wise to reach out to your bank and see if they have any partners in Italy, for example we found that Bank of America has an extremely low exchange fee at the BNL ATM's that are very common across Italy.
Italy has a very low violent crime rate and while safe, the country is known for petty theft. Though we did not experience it, cities in Italy (especially Rome) have reported high counts of pickpocketing, especially in crowded places such as on subways. Be sure to keep bags zipped and where you can see them at all times, and you should not have an issue.
While wait times for new and updated passports are currently quite long, you should have plenty of time. Go ahead and schedule an appointment to get your documents in order as soon as possible! https://travel.state.gov/content/travel/en/passports.html
Be sure and call your cellular provider ahead of time and let them know of your travel plans to avoid any outrageous international fees.