The latest stable version of Backbone.js is 1.3.3, and it was released on April 5, 2016.
As of my last update in January 2022, the latest stable version of Backbone.js was 1.4.0, released on September 15, 2016. However, it’s important to note that software versions may have been updated since then. I recommend checking the official Backbone.js website or repository for the most up-to-date information on the latest stable version and its release date.