Has Anyone Here Been In Florence During The Feast Of Saint John?

Yes, it will be a festive day, much like a Sunday.
The Duomo area will be closed as there is a solemn mass at 10.30am, preceded by a cerimony at 10am in the Baptistery. Parades from different quarters meet at Palazzo Vecchio where they hommage the mayor; the mayor, preceded by the city trumpet players, will walk to the Duomo for the Mass.

In the afternoon there will the the last play of the historic soccer tournament - if they get to play it as sometimes the tournament turns too hard and gets suspended. At 10 or 10.30pm there are fireworks from Piazzale Michelangelo, traditionally people meets along the Lungarni to watch them.

It sounds very festive! That should be fun for my son to witness. Thanks for your reply.

I was there last June, and saw two parades through the Piazza della Signoria on two consecutive afternoons - historical costumes, horses, music, flag-tossing and catching. You'll love it. The second day, I had just left the Uffizi, so they definitely don't close everything.

The original poster's question (one of them) was:
Is everything in Florence closed on the Feast of St. John?

To assist the OP: Does anyone know the answer to that question?

So far, we have only learned that the Uffizi is (apparently) open.

Come to think of it, the first day I had just come from the Brancacci Chapel - also open.

Roberto, where are you?
Please enlighten us on the OP's question above: What is the extent of closures in Florence during the Feast of St. John?

I wrote: it is a festive day, much like a Sunday.
Are shops open on a Sunday? Mostly not. The same on St. John's day.
Are restaurants open on a Sunday? Mostly open. The same on St. John's day.
The same for museums.

We were there for for the feast about 10 years ago. There was a spectacular fireworks display along the Arno on the evening of the feast. I don't know if it is an annual thing but I would expect so. It was great fun.

Like any Sunday or holiday. Shops mostly closed. Bars and restaurants are open.
If you prefer weekdays, just cross the city limits to Fiesole, Scandicci, Sesto, Bagno a Ripoli, Impruneta, and you are back on a regular week day. It's a holiday only within the city limits.