Florence - Feast of St. John the Baptist

Not much if anything, museumwise, looks to be closed on Wed. the 24th. At quick glance, the Uffizi, Accademia, Pitti, San Marco, Bargello and probably others I didn't check are open.

http://www.b-ticket.com/b-Ticket/uffizi/default_eng.aspx

The duomo and most of its associated structures are also listed as open (the crypt is closed). There looks to be some parts of the civic parade and mass that will affect morning visiting hours on the 24th so maybe just do that one on the 25th or the afternoon of the 24th. I would expect some street and piazza closures here and there (see the 2018 piece linked below).

https://www.museumflorence.com/plan-my-visit

While from 2018, I don't imagine the festivities change much

https://www.visitflorence.com/florence-events/june-24-feast-day-in-florence.html

Also:
http://www.welcometotuscany.it/itineraries/june-24th-in-florence-san-giovanni-s-celebrations.html

The Duomo will have a solemn Mass starting at 10am at Baptistery, then the Archbishop moves to the Cathedral around 10.30, followed by representatives of various congregations and associations, then the mayor accompanied by the city trumpet heralds. It is a nice and interesting ceremony, that will last till 0.30pm - no visits to the church during the Mass. You may wish to attend but without some connections and invitations for the front files you are not going to see much.

The day has the same festive status of a Sunday, for Florence only (and for the hundred or so cities and towns that have St. John as their patron saint). Trains (national) are running on ordinary schedule, but buses (local) will be running on festive schedule.

Museums will be open. Not much closes except for shops and offices located within the city limits. Restaurants are open.
But all you have to do is cross the city limits into any of the surrounding municipalities and it will be a working day like any other. It will be a perfect opportunity to visit Fiesole, the Etruscan town that predated the founding of Florence. Or you could go to Sesto Fiorentino (the famous fairy tale of Pinocchio was set in Sesto) and visit Ikea.

the three big days for closures of museums and tourist sites in Italy are Jan 1, May 1 and Dec 25. They don't close for other holidays although it is always wise to double check.

We were there at this time many years ago and enjoyed a lovely fire works display on the banks of the Arno. Assume it is annual, but of course double check.

Yes. Fireworks happen every year on the 24 of June.
The shoot them from Piazzale Michelangelo and the best viewing spot is along the Arno river more or less between the two bridges in front of the Piazzale (Ponte Alle Grazie and Ponte San Niccolò).
It also the day when the Final match of the savagely violent Historical Football is played. If you think that American Football or Boxing are too violent, don’t go to the game, because you will see both in display in the field, except that players have no protection gears and in the past players have died or were severely injured. Many of the players are former convicts or even current convicts on parole.

Thanks everyone! I appreciate the insight and tips. So looking forward to visiting Florence again.