https://www.gamespot.com/articles/hearthstone-giving-free-packs-for-daily-quests/1100-6457586/

Another Hearthstone expansion is on its way next month, and Blizzard has announced a pretty big promotion to lay the groundwork for it. Starting on Monday, March 26, every daily quest you complete will grant you not just your usual gold reward, but also a card pack.

According to the official blog, you'll get a randomized reward pack from one of four expansions: Journey to Un'Goro, Knights of the Frozen Throne, Kobolds & Catacombs, or the upcoming Witchwood. The first three are all from the current season, so they won't rotate out of Standard in a few weeks like some older sets. The Witchwood packs will sit in your collection and can't be opened until the expansion launches.