This is the list of Florence State museums that will be open on April 13 (“Pasquetta” or Angel’s Monday) as advertised on the official site.
Many of these are generally open on Sundays so they’ll be open on Easter Sunday. Check the site above for hours and to double check which ones will be open on Sunday.

apertura ordinaria – would normally be open on this Monday anyways
Cappelle Medicee
Museo di Casa Davanzati
Giardino di Boboli, Museo delle Porcellane, Museo degli Argenti and Galleria del Costume (biglietto unico, a common ticket for these museums)

apertura straordinaria – normally closed on Mondays
Galleria degli Uffizi
Galleria dell’Accademia
Galleria Palatina e Galleria d’Arte Moderna (biglietto unico, a common ticket for these museums)
Museo di San Marco
Museo Nazionale del Bargello
Villa della Petraia
Villa di Poggio a Caiano