Minecraft wiki entry for Combat_Update, associated text: Mojang released the first pre-release on February 17, 2016. The date for this was hinted at by the image in the blog post that announced snapshot 16w06a. The data values of the wool in the image were (1, 7, 0, 2, 2, 0, 1, 6), having a possible (though never confirmed) interpretation of "17-02-2016", a date just one week from 16w06a. The second pre-release was released on February 18th, the third pre-release was released on February 24, and the fourth pre-release was released on February 26.