From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sevii Islands are a archipelago of nine islands and other islands surrounding them east and southeast of Kanto. They have appeared only in the FireRed and LeafGreen versions.
In FireRed and LeafGreen, they are accessible from the Vermillion City harbor via a system of boats called the Seagallop Ferries. To be able to access the Knot, Boon, and Floe Islands, one needs a Tri Pass, received from Bill. To be able to access any of the islands except Naval Rock and Birth Island, one needs a Rainbowpass, received from Celio in One Island. To be able to access Naval Rock, one needs a Mysticticket, and to be able to access Birth Island, one needs a Auroraticket, both of which can be obtained promotionally with Mystery Gift.
According to an old woman on Quest Island, they were named the Sevii Islands because they were made in seven days.