On Saturday, Everett will hold a voter referendum to decide if Wynn Everett, a $1.2 billion resort casino proposed by Las Vegas developer Steve Wynn, should be built on a former Monsanto chemical factory on the Mystic River. The referendum is required by the state’s gambling law. The law also requires a host community agreement to address the impact of the development. Everett voters will not be voting to approve the agreement; they will be asked to vote “yes” or “no” on whether the casino should be built in Everett. If approved, Wynn’s application may advance before the Massachusetts Gaming Commission. If turned down, the development cannot be built. Wynn is one of three developers vying for the one resort casino license available for Greater Boston. The commission plans to award the license next year.