The theme for this literacy day is
"Literacy and Sustainable Societies"

It is significant in a way that United Nations adopted Millennium Development Goals which aims for more inclusive growth. 
It is inferred that Sustainable development would be a prerequisite for it. So this year theme focuses on Literacy and Sustainable societies.
Celebration of the 2015 International Literacy Day  will happen for 2 days on Sep 8, 9 at UNESCO headquarters in Paris.