Celebi was once available to Pokémon Crystal owners in Japan, who connected to the Mobile System GB and used the News System at the Pokémon Communication Center, but it was time-limited, much like a Mystery Gift event. This event was coded into international versions of Pokémon Crystal, but was unfortunately left unactivated, leaving players outside of Japan — as well as any Japanese players who missed out — with no way to obtain Celebi for themselves.
But rejoice — the Virtual Console editions released on the Nintendo eShop enable this event for everyone! What’s more — this Celebi can travel across time and space with you via Poké Transporter!
To get Celebi for yourself, visit the Goldenrod City Pokémon Center (or the Pokémon Communication Center in the Japanese games) after defeating the Elite Four. Upon leaving the building, an attendant from the second floor will approach you and hand you a GS Ball.
Take the GS Ball to Kurt, who will take a day to examine it. When you speak to him the following day, he’ll notice some strange occurrences from Ilex Forest, ask you to investigate it, and return the GS Ball. You should take this opportunity to prepare by purchasing some Ultra Balls, Potions, etc.
Visit the shrine at Ilex Forest. You’ll need to enter it via Azalea Town because the north entrance will be blocked off. You’ll also need to bring a Pokémon with Cut to clear the tree in front of the shrine. Be sure to save the game — if defeated, Celebi will never appear again (by the way, it isn’t shiny-locked). This Level 30 Celebi knows the moves Heal Bell, Safeguard, Ancient Power and Future Sight.