West Yellowstone Chamber of Commerce issued the following announcement on April 9.
The Yellowstone Airport outside of West Yellowstone, Montana opens for the season on May 9. This is two weeks earlier than in previous years. The season will also be extended two weeks in the fall until October 13. Flying right to West Yellowstone saves you time. You can get right into Yellowstone National Park or into the Custer-Gallatin National Forest. Ease and convenience await you when you choose the Yellowstone Airport
Original source can be found here.
Source: West Yellowstone Chamber of Commerce