The b-ticket website for the Uffizi and Accademia indicates some later hours during high season:
http://www.b-ticket.com/b-ticket/uffizi/default_eng.aspx
But info for the Uffizi isn't consistent.
Click on the blue headline text in the box for the Uffizi and and info box pops up with this line under "Opening hours":
"From 5 June to 25 September (with the exception of August 14) extended open time: 8:15-22:00"
But clicking on "Buy your tickets" and then a green "Tickets" box brings up this text:
"Every Tuesday and Wednesday from 5 June to 26 September extended open time: 8:15-22:00"
As a quick look around the web shows Viator peddling an "Uffizi by Night" small-group tour on Tuesdays and Weds. only (from June 5), it looks like extended hours apply only to those two days of the week.
Extended hours stated for the Accademia appear to be consistent:
"Every Tuesday from 5 June to 25 September extended open time: 8:15-22:00"