Chicago must hold a mayoral election next year to fill the two years remaining of the term of the late Mayor Harold Washington, the Illinois Supreme Court ruled Monday. The high court said that a reading of state election law, along with the state law governing municipalities, requires that a special election be held to fill a vacancy that occurs with more than 28 months left on a four-year term. A lower court already has called for a primary Feb. 28, followed by a general election April 4. Washington, the city's first black mayor, died of a heart attack Nov. 25, 1987, with 31/2 years left of his second term.