From Bulbapedia, the community-driven Pokémon encyclopedia.
Lake May (Japanese: ハルカレイク Haruka Lake) is an anime-exclusive location in Hoenn, found in between Mauville City and Fallarbor Town.
It appeared in Love at First Flight. After Ash and his friends stumbled across it, they found Romeo and Juliet training their Volbeat and Illumise for a festival that was set to begin soon. However, Romeo was having problems, and fell into the lake at one point. Fortunately, Ash and the others rescued him.
With their help, Romeo was able to successfully train his Volbeat, and was able to show Juliet his true feelings for her.
Pokémon seen at Lake May