We are adding Ashe and Garen temporarily to the free-to-play champion rotation for a period of approximately two weeks in North America only, beginning on February 23, 2016 at 4:30PM PDT. This is one of several tests to try to improve the onboarding experience for new players. Thank you for your support as we look to make learning League of Legends easier.
EDIT: Ashe & Garen will not be taking the place of any other champions in the rotating pool; they will be additive. :)