If Leeching Lurker is on the battlefield during night, and the active player casts two or more spells during the turn, it will become day as the next turn begins. This will cause Leeching Lurker to transform into Curse of Leeches and become attached to a player. This all happens before that turn's upkeep, so the triggered ability of Curse of Leeches will trigger at the beginning of that turn's upkeep.