Uffizi and Accademia: tickets and booking

Except a few small museums, that are only open by appointment, or museums that are hosting an event or a special opening, generally no reservation is required to visit the museums of Tuscany and tickets can be purchased on site, at the time of the visit. 

Two "heavyweights" are an exception to what has just been declared, and when it comes to them, reservations are not compulsory, but definitely highly recommended: Uffizi Gallery and Accademia Gallery, in Florence.
The line in front of these two museums begins to form early in the morning, even before the ticket offices open, and during high season it can reach a daunting length.
From April to September, or during particularly overcrowded periods (Christmas, Easter), the advice is to book, always, no matter the day of the week. During the rest of the year it's a good idea to book at least for visits during the weekend. 

Here are the information on how to do it, through official sales channels.