New allies have entered the fight for Azeroth, and you're needed on the frontlines. This weekend, we're upgrading all inactive WoW accounts to allow you full access to the game and all of your characters without a subscription. You’ll also be able to access the Battle for Azeroth™ trial experience, even if you do not own the expansion.
Source: mydealz
I heard only 3 free hours will be given during this period.