Blizzard will be unlocking the full Heroes of the Storm roster from January 13-16 in celebration of a big 2016 and to start off 2017 on the right foot. Last year featured a number of brand new events and the addition of a lot of new heroes. A free roster weekend offers an opportunity to try out all those heroes that haven't come up on the free rotation lately, including the wickedly OP Ragnaros!
Source: Heroes of the Storm site
