It’s open access week this week, 23-29 October 2017.
For example, all Royal Society journals are open access during this week:
Royal Society: “It’s Open Access Week and, to mark the occasion, all our content is free to access until 29 October. Our extensive archive contains articles by Sir Christopher Wren, Benjamin Franklin, James Clerk Maxwell, Alan Turing, Kathleen Lonsdale, Stephen Hawking and Dorothy Hodgkin. And, of course, all our recent cutting-edge content is in there too.”