Arriving a whole day early necessary?

Perhaps this your first transatlantic trip? The six hour jet lag and the dreadful night's sleep on the plane should not be minimized. Personally, I'd arrive a couple days early so I can fully enjoy the tour I paid for.

I would fly in a day early. Last May, my flight from DTW (Detroit) to JFK with a connection to MXP (Milano) was fully boarded when the flight was cancelled. Flights had to be rerouted, causing an 11-hour delay in arriving, and adding an extra connection.

About two dozen passengers on the original flight missed their tour's first day due to the rerouting.

If you experience jet lag, that extra night in Europe will help you adjust.

You are spending good money for your tour, don't nickel and dime yourself now. Believe me, no one on this forum likes to spend money unnecessarily, but arriving a day or two before is money well spent.
A group of us were going on a Baltic cruise out of Copenhagen. One person coming from Florida was delayed due to a storm in Virginia, and missed her connection in JFK. if not for her travel agent getting her on another flight, she would have missed the cruise. Instead if coming in the day before, she arrived that morning. The cruise was leaving on a Monday, some of us arrived Saturday, a few others Sunday, and she raced in all stressed out that morning. Not a good way to start a vacation.

Voting with the others: I'd definitely arrive a day early for all the good reasons already listed.
Besides needing arrival day to adjust to jet lag, flight delays/cancellations are not uncommon enough to take the risk of missing out on first-day activities.

It is not necessary but as others already told you it is wise.

In 2012 we arrived at 8 am and has nothing scheduled until a tour meeting at 5:00 that evening. We left our luggage at the hotel, grabbed a cab, and walked into the Pantheon just after they opened the doors. We spent the rest of the day wandering the Centro Storico, sometimes getting lost, and stumbling across sights I'd read about since I was 10 years old. We had a wonderful lunch in Trastevere, rested in the shade of the trees in the Capitoline Hill, then made our way back to the hotel. After our tour meeting, we went back into town and dinner in the Piazza Navona before finally catching a can back to the hotel. I'm pretty sure we were so tired we hallucinating. It was also one of the best days of my life, so far.

If you're seriously pinching pennies, you may be able to find a less expensive hotel for that first night. It will be a bit of a pain to relocate to the tour hotel the next day, but not nearly as bad as being jetlagged at the beginning of the tour or potentially missing the first day because of a significant flight delay.

most people (not all) suffer from the jet lag for at least a couple of days after arrival. Its not about catching up on sleep, its about adjusting your body's clock to a different time zone. Spending the first days of your tour in a dazed condition can be minimized if you dont have to hit the ground running. On our first tour, we noted how people who arrived the day of the tour start were nodding off into their plates at the meet-up dinner.

Also note that you will likely not be able to check in to your hotel until the afternoon anyway, so that can add to the arrival day / first-tour-day stress.

Also note that you will likely not be able to check in to your hotel
until the afternoon anyway, so that can add to the arrival day /
first-tour-day stress.

True although most hotels will keep your luggage and provide a public restroom for a quick wash of face and hands before setting out to explore prior to the check-in hour.

Arriving a whole day early necessary?

Not "necessary", but if you do not, you have a very good chance wishing you had.

Thanks everyone for the great advice! Extra day it is!

I was going to throw in my two cents, but apparently everyone else already persuaded you to arrive a day early. FWIW, I think that is an EXCELLENT decision, and you'll get more from your tour by padding it with that early day.

For anyone else wondering . . . .

You leave New York at, say, 9 pm and you land in Rome at around noon so it sounds like you have a whole night to sleep. But the plane is in the air for about 8 hours, including drink and meal service after takeoff and probably another meal service before landing. It's 30-45 minutes before they start to serve the first go-round and another 30-45 minutes to landing after they clear up the final service. If you have 5 hours of quiet time, that's a lot.