The Chicago Teachers Union says members will walk off the job on Sept. 10 unless a new contract is reached with Chicago Public Schools.
Thursday's announcement comes one day after the union issued a 10-day strike notice.
Union President Karen Lewis said both sides are far apart, with teachers worried about issues that include wages, job security and a new evaluation system.
It would be the first strike in 25 years for the nation's third-largest school district.
Lewis said the union will continue negotiating until there is a contract.
Some of the district's 400,000 students have started school; the rest begin on Tuesday.
Chicago Public Schools has announced a contingency plan if teachers walk out. It includes opening 145 schools, staffed by district employees and volunteers, for four hours daily.