The Lily of the Valley Conference in Pokémon the Series: Diamond and Pearl is held in the Sinnoh region. It is the Sinnoh League tournament and serves as the major competitive climax of Ash’s journey through the region.
Ash qualifies for the conference after collecting the region’s eight Gym Badges. The tournament takes place on Lily of the Valley Island, a location associated with the Sinnoh League. Ash defeats several notable opponents there, including his longtime rival Paul in a full battle.
Ash eventually reaches the semifinals, where he faces Tobias. Tobias uses the Legendary Pokémon Darkrai and Latios, and Ash becomes the first known trainer to defeat Tobias’s Darkrai. Ash loses the match, ending his Sinnoh League campaign before the final.
Sinnoh is the regional answer, not Lily of the Valley. Lily of the Valley is the island and conference name, whereas Sinnoh is the broader setting introduced by the Diamond and Pearl games and anime series.