https://www.gamespot.com/articles/last-chance-to-get-this-free-pokemon-in-ultra-sun-/1100-6461799/

Last year, to commemorate the release of Pokemon Gold and Silver on 3DS Virtual Console, The Pokemon Company gave players who purchased either title a free gift: a download code for the Mythical Pokemon Celebi. If you've yet to pick up one of the classic games, you can still do so and receive the free code, but you only have until the end of this week.

The free Celebi code will be given out with a purchase of Pokemon Gold or Silver until this Friday, September 21. This offer only applies to the Gold and Silver versions; Pokemon Crystal, which released on the 3DS Eshop several months after the other two Gen 2 titles, doesn't come with a free code as you're able to catch Celebi in the game after completing the main adventure.