Directly to you from Twitter:
Tempest Trials: A Gift of Peace has begun! Make sure you claim your rewards from these quests each day. You can even get Refining Stones for Weapon Refinery as rewards! #FEHeroes pic.twitter.com/5vBNDd3sQL— Nintendo of America (@NintendoAmerica) December 26, 2017
…Wanna play? Buy a 3DS.
That being said, click on Fire Emblem Heroes for everything you need to know about the game. 😀